Well, I'll tell you a few things.
Don't BOTHER with QuakeLive. I have had problems in the past with the PPC macs and QuakeLive, but if you have an Intel Mac laying around it should do fine. The reason why you're getting an MSI is because your mac is PPC I believe, I had the same problem but it downloads just fine on my Intel.
Openarena is nice; Better than Quake 3 actually IMO. Why, you ask? LAG COMPENSATION!!!!
But, since you're bent on getting Quake 3 like I was a half year ago, I tell you what. Stop trying to pirate the thing, it's frivolous and in my experience, 90% of the time never works. Don't do it.
Instead, go and buy a cheap used copy. And when I say cheap I mean CHEAP. Hell, I got the entire Quake Trilogy used for under 20 bux. Then, heres what you do. You go and get the Quake App (Compiled for PPC; Quake Universal Binary I've had trouble with on G3's but maybe if you have a G4 it might work)
I have run into difficulty finding the PPC exclusive Quake app, so if you find that you need it because the UB isn't working I can give you a drop box link. I must warn you though, in order to use the Quake App you need to have a PC install first... hope you have a Windows laying around.
So, TL;DR:
1. BUY a used copy
2. INSTALL the used copy on a PC
3. GET the Quake App
4. Profit!
EDIT:
It works for mac im on it now make an account sign it and it downloads this QuakeLivePlugin_491.dmg plug in and just go from there!
Don't get his hopes up. It doesn't work on PPC macs, at least from what I can tell, since he got an MSI instead of the DMG.