An XNA premium membership ($99/year) is required. You don't need to purchase Reaper if you just want to review songs, but you do need to download the free Magma program.
As for doing that right now, the RBN is still in closed beta, so you'll need to wait for the elusive open beta. When it will eventually come is anyone's guess.
As trg007 said, you'll need a XNA Pemium Membership and Magma. You will also need to have an Xbox 360 on the same network as the computer running Magma with a copy of Rock Band 2 that has the latest title updates downloaded (which includes Audition Mode under Extras in the main game menu). Playstation 3 and Wii users aren't elgible...yet.
How do I become a peer reviewer?
Linear Mode
