It could be possible you need either a AWD(All Wheel Drive), Back Wheel Drive or a Front Wheel Drive vehicle. If that ain't the case, it could be the class level, cars range in class from F, E, D, C, B, A, S, R3, R2, R1, and X. Also they have certain cars that are to be used for events, it should list that right off on the menu of the event requirements.
Here is a easy way to obtain 100,000 plus credits fast. Get with a couple online friends. Setup a race for 30 laps, no matter the length(I did this on a 3.63 mile track). The winner and 2nd place will receive over 100,000 credits, 3rd place will receive around 50,000 to 100,000 credits. Races will probably last around 30 to 45 minutes but it's fun and worth it.
I cannot give you credits but I can let you in on this, get with a couple friends and race a 3.5 mile track with 30 laps, winner will get over 100,000 credits. Fast easy way to get them, specially if your better than your buddies.
No I'm sorry there are no cheat codes for the game that can allow you to do that. Unless you used a JTag Xbox, which is illegal and can get you banned from Xbox Live.
If you talking about so you can play with other people, that's not how it works in this game. While just roaming around it should give you a notification that a player is near you and give you the option to join them. Another way if you want to play with a friend, have them in there game and you can go to there profile and select "Join Session in Progress".
The only other menu is the main menu that I can think of and it brings you to that every time you fire up your game.
The coop mode is done by Xbox Live, while playing you will be notified of players in the area and it will give you the option to join there adventures. It's not to complex, rather easy.
Using a Jtag Modified Xbox system can give you all gold guns, other than doing that you will have to work up the guns yourself. Using a Jtag system will however get you banned from Xbox Live so I don't recommend this. Other than doing the Jtag or working your guns up yourself there isn't a hack for it.
Once you are so far along into a Zombie level, I can remember getting one on level 25. Just in killing the zombies is how you get it, just like when a Max Ammo appears or Instant Kill, it's the same way for the Death Machine.
Sad part about prestige really, it isn't possible in Black Ops, prestige makes you lose everything. Modern Warfare 3 introduced a prestige token that you can earn for prestige and you was able to unlock items and keep them through future prestige levels. Maybe Black Ops 2 will have this option, but as for Black Ops, it just ain't possible.
The only other way to do that is using a Jtag Xbox. Which is a modified system by hackers and is very illegal and will get you banned from Xbox Live. I don't recommend any cheating for that. It just ain't worth it.