Did you try chmod-ing it?
chmod u+x /Applications/Tremulous/Tremulous.app/Contents/MacOS/Tremulous.ub
The above command assumes your Tremulous folder is called "Tremulous" and is in Applications.
You should try changing CFBundleExecutable in Info.plist to "Tremulous.ub" (without quotes), instead of renaming Tremulous.ub itself.
OK, I deleted the copy of my trem folder (I had copied it, renamed the copy to the original name, and renamed the original something else, so all's expendable), remade it (in the same way), and entered that command, but there was no
CFBundleExecutable in
Info.plist. There was
CFExecutableName and
CFBundleName, and I changed those to "tremulous.ub" and it runs now, but just base Trem, not TremFusion.
Also, what does that command do? And it's entered in Terminal, not Tremulous' console, right?