Hi. Welcome to the forums, and to Tremulous.
The forums have a Troubleshooting section. Seeking help there usually will be faster than in General Discussion.
I believe the answer to your question goes something like this:
Try dragging and dropping them to the same folder where all the extra maps go, which, iirc, is the following path -
Mac: ~/Library/Application\ Support/Tremulous/base/
- where ~ stands for the user-defined part of the string (you).
In other words, look for a second Tremulous folder called "base", in the Library for Applications Support. Open Finder. Click on "Library". Now click on "Applications Support". Now look for a Tremulous folder. Click on that, and you will see the constituent folders for Tremulous add-ons. Drag the maps to the "base" folder that you see. Now boot up Tremulous, and have fun!
P.S. There's probably a way to make Safari put them in the correct directory, but it does weird stuff sometimes, so at least having it leave the maps on the desktop makes it easy to find where they went.
Cheers!