Near the top left-hand corner of the forum, under the path that spells out what forum you are in, there's a link that says "User CP". Click on that. You'll see a list of options down the left-hand side under "Your Control Panel". Under the section for "Settings & Options", click on "Edit Signature". It will bring up an editor box just like what you use when you post a reply or a thread. You can type a signature, and a link to your report in there.
If you're not linking to a report posted elsewhere on the DIS (ie, it's on an outside site) make sure you check out the guidelines for posting links in your signature.
http://www.wdwinfo.com/guidelines.htm/
Let me know if you need more detail than that.
Sayhello
ETA: OK, duh, once I posted this, I realized you already have stuff in your signature, so that wasn't the question. When you get to the editor box, there's a button above it that looks like a little tiny picture of a mountain & the sun. Click on that, and paste your link into the box. That should do it. If you keep having issues, it may be something incompatible about the site you're trying to link to. You might also try putting it above your ticker. There's a physical max length to your signature, and the link may be farther down than is displayable. You might have to consolidate some of the lines of your current signature. This is all just guessing since I don't know what you're trying that isn't working.