If you are referring to the 20 Free Songs, these songs only came exclusively with the purchase of Rock Band 2.... These songs came on a Redeemable code on the back of the manual inside of the Rock Band 2 case. If those are the free songs you are referring to, that is the only way to redeem those songs...
The only other free songs are Still Alive,Headphones On, Promised Land, and maybe one other I forget. Those are available in your respective systems music store.
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