It's really easy. Just go to your site and create your countdown following their steps.
After you've created it, you'll see several different codes. The one you want to use (copy) is the first one, which is labeled BBcode (which stands for Bulletin Board code)
If you click inside the box with the code, everything there should turn to a blue background. If it doesn't all turn blue, then after you click in the box, enter CTRL-A on your keyboard. (This means to hold down the CTRL key, and then press and release the 'A' key, then release the CTRL key. This selects ALL text)
Once everything is blue, you want to copy it to your computer's memory. Do this by entering CTRL-C (just like you did for CTRL-A). CTRL-C means to copy.
Now go to the DISboards. Near the top, and on the left, you'll see what says "UserCP" (This stands for User's Control Panel). Click that. You'll get a dropdown listing. One of these will say "Edit Signature". Click that one.
You'll see a screen with a box similar to what you get when you're making a post. This is where you put the codes you want to use in your signature. Click anywhere inside the box. (This selects it). Then do a CTRL-V in a similar manner as you've done before. CTRL-V pastes the code from your computer's memory to your signature.
Go to the bottom of the page and click to preview your signature. If everything looks correct, then at the bottom just save and exit.
Some additional things: If you want to put your name in your signature, just put it inside the box like you would for a posting. You can make it bold, larger, or color, just like any other text you might put in a post. Anything you put in your signature box will show up on all your posts. (There is a limit of 250 pixels vertical space, so if you try to put lots and lots, and it exceeds the limits, then it all won't show. You can look at others' signatures and get a feel for how much you can have.
Hope this helps.