I did a few things to your post here - split it off the topic in the 'mapping' forum for one, and changed the subject to something useful :>
You found the Tremulous forums, that's great. But please don't post in a completely unrelated thread about a problem you're having, it's likely to be skipped over as others who might be able to answer the problem won't see it (ie, if someone doesn't care about mapping, why would they read posts in that forum?)
As for your problem, you mention you're using a G4. There's a known issue with PowerPC Macs and selecting the grenade or the jetpack - in short, they don't work. The way around this is to bind a key to use them. Open the console (drop it down with the '~' key) and type
/bind g itemact gren
/bind j itemtoggle jetpack
This will allow you to throw the grenade with the 'g' key, and to toggle the jetpack on and off with 'j'. You can get fancier with binds if you like - there's a lot of customization available - and maybe do something like this:
/bind g "say_team ^1Grenade!; itemact gren"
Now when you hit 'g', not only do you throw a grenade but you also warn your teammates that you're doing so. Why is this helpful? For one, friendly fire servers mean that grenades can harm teammates too, so it's always nice to give them a heads up. But just as important, if you're on the alien team you can use the same key to tell your teammates when you see a grenade - even though you can't carry one as an alien, so the itemact does nothing, the say_team still does and your teammates can be warned about that cute little pineapple laying on the floor.
