Getting Holdem Manager for Free is easy:

Sign up for an account at RakeBackNation.com using this link
Sign up for a rakeback account at any of the rooms RBN offers (dozens to choose from).
Rake a minimum of $250 total – it can be at one room or combined across several rooms (you can check your stats, updated daily, in your RBN account).
Email support@rakebacknation.com with your account details and you’ll get your free license for Holdem Manager.

FAQ

What is rakeback?
Rake is a small amount of money that poker rooms take out of each pot – that’s how they make their money. Rakeback is when poker rooms rebate some of that money to players. It’s one of the ways that online poker rooms ‘comp’ players – if you’ve played live poker, it’s just like when rooms give you meals or points based on how much you play.
What rooms offer rakeback?
Almost every major room offers some form of rakeback.
Is rakeback ‘allowed’ by the rooms?
Absolutely. It’s a way that rooms promote themselves, especially to high-volume players. They only offer it through sites like ours instead of offering directly because if they offered it directly they’d have to give it to everyone.
How is my rakeback paid?
Directly in to your poker room account, generally on a weekly basis.