Addons folders are virtual, don't go adding stuff to /addons unless you know how to make an addon.
You'll need all files associated with the model, and a special txt file telling gmod 'this is a legit addon'.
Since whatever you're trying to use isn't already in addon format, copy the files you have to your gmod server's models folder, in thier own folder to make them easier to separate from default files.
/models/somefolder/file.mdl
models/somefolder/file.vtf
It's explained reasonably well within our faq.
http://forums.ulyssesmod.net/index.php/topic,3072.0.html