Yeah, I kinda figured it had to be relative.
I'm not suggesting that you don't fix it, but determine what the problem is first. Moving it to C somewhere and running it will verify whether it is a security issue or if it is a path issue. I'd suspect a path issue nonetheless now that you verified that the manifest has to be relative.
So, there are two ways to fix this. First, you can add a bootstrap path to load the jar first using -Xbootclasspath/a:/path/to/ur/resource.jar, and the other is to move the external jar (what I would recommend). Grab your external jar and throw it in your jdk's lib\ext directory. Not sure where you'll find it, but it'll be along the lines of C:\Program Files\Java\jre6\lib\ext for example. Once there, that should run no problem with a manifest of just Class-Path: yourexternal.jar