Ah, so you want to upload new design ... hmm, how do I do that? Hang on, its very easy.
First, we assume that you have already registered as a member - oh, not yet? Ok, either go directly here and create an account for yourself or you can check out this tiny HOWTO to learn the details of creating an account.
Right, so now you have an account and you have logged in (noo, I am not going to make a "logging in" HOWTO..).
Click on the "Upload" link at the top of the website and you should see a page like this:

As you can see, there is a field for you to fill in a name for the design - this is for your reference and can be any name. After uploading this is what the design will be called in the list in your account; you can also change this later. So, fill in a name (eg. "arc box 1" or "arc_box_r1") and then click on the button to select a file which will prompt you to select a zip file or tarball containing Gerbers.
Ahhh, zip file? Tar ball? Hmm, ok - if you are stuck now you can check out this HOWTO to learn how to create a zip/tar file with your Gerbers. This will be a compressed file (my_design.zip or my_design.tar.gz) that has the Gerber files exported from your CAD/CAM software. The files must be in the root or base of the zip - not in a subfolder! When you create the zip select a folder and zip that, select the individual files and zip them together.
Ok, next step - you have selected a zip file with Gerbers in it, named it and clicked "Upload" and now you should see something this next:

These images will hopefully show each of the layers in your design - and if you used a normal naming convention they should already be pre-selected to the correct layer. Please see this FAQ for more on what a Gerber file is and what the normal naming convention for extensions is. If you did not use the normal extensions (some CAD software does not ..) the layers may not be matched up; no problem - you can adjust them here.
If the images look a little funny, not to worry: these are only for reference to make it easier for you to select the correct layer for each file, we don't use them for layout or production.
After you have checked and/or selected the files for each layer you need (you can leave them "Unused" for extra files that you don't need), click to continue at the bottom of the page - if the page doesn't change then scroll up to see if you have assigned the same file twice or something like that.
The next page will show you a size estimate for the design - this is the area for which we will charge if you order the design for production:

If you find an error here (eg. size is incorrect) you can contact support to resolve it. Note: it is important that you have only the design data in the Gerbers and not extra text or other things outside of the design outline. If you have extra things outside of the design the sizing code may assume that it is part of the design and the estimate will include the area used. You can check to see what the design looks like in a viewer like gerbv or ViewMate - if you see text outside of the outline of the design this may cause sizing errors.
If everything looks ok now you can click "Continue" - or "Cancel" to abort the upload. Next the DRC Bot will fetch your design and have the DRC code check for basic bloopers - Note that this does not guarantee that the board will work, but it will make sure that your traces/spaces are not too small to manufacture. The Bot will send you an email with the results of whether the design passed or failed - the email will also have images for reference and if there were problems the images will highlight the areas in pink or orange (top.png and bottom.png). : pink means the trace is too small orange means the space was too small. You can also check the status in your account under "PCB Designs".
Usually you receive an email in a few minutes - if it is a very large or very complex design it can take as long as an hour for the DRC to work through it. Occasionally it fails, in this case you can contact support and we will look into it. And, the DRC is ok but not brilliant - it gets confused by some output; if you really think that the design is ok please do contact us, we can have a quick check and pass it manually if need be.
That's it! Your new design has been uploaded and checked.
If the design has passed you can either follow the link sent by the Bot to add it to your cart or you can do so directly from your account. You can order as many copies as you like as many times as you like. From "PCB Designs" in your account you can also delete old designs, rename them and set whether the general public can view the designs or not. There is also a link created on the bottom of the design view page (click "View" in the list of designs) that you can send to a friend to allow them to view or order the design - just copy the link into a browser and it will go to the page for that design. Note that the link is only created after you add the design to your shopping cart at least once (you do not need to complete the order). You can also change the name, model, permissions, etc. of the design on the design view page.