A. On the Foundry mission browse window in game there should be a button that says become a reviewer. Just click that.
B. No. There are no rewards at all (since now mission below 5 plays qualifies for rewards) besides the satisfaction of helping out new authors or missions that are struggling to get five plays
However, it is really essential to sign up if you're an author, since you will need to see your own mission right away after you publish it, and you won't be able to if you're not a reviewer.
The Foundry Roundtable live Wednesdays at 7:30PM EST/4:30PM PST on twitch.tv/thefoundryroundtable "I would rather take a chance on getting a change made to the game through positive engagement than know for sure that I got a change made by complaining" -drogyn1701, May 14, 2014.