You must have the Spoot the Magic Llama cheat out.

Press the top button on the left of your Xbox 360 remote and then hover it over the mailbox outside your house.

It will give you a bunch of options. Click choose job or something like that and it will give you lists of careers. Chose any one you want!

But this is for Rockstar, so click on music career and go down and click on the second to last option, then scroll down and if it says Rockstar click it. If not, go back again and click on the previous last option and go down to the bottom again.

Rockstar should be available!