Please be more specific, I think you mean downloading maps, correct? To do this:
1. Run Tremulous until your at the Start screen (Screen with the moon background and awesome(?) music)
2. Click the tilde (~) key on your keyboard to open the console (aka, the big black box that comes down on your screen)
3. This thing lets you type commands (text) into it and lets you execute shit. Now type in:
sv_allowdownload 1
Exactly as shown above, no caps and yes, there is a space between sv_allowdownload and 1.
4. Try joining a server with a custom map, it should work, but it'll be as slow as hell to download. Try finding an updated Tremulous client by using the forum's Search function as shown above in this page.
Cheers. If this isn't the answer your looking for, reply back and specify your problem in detail.