How do I unlock bonus powers?
You can learn a single power in the Normandy Medical bay. Learning a new one will replace the old power. It costs 5000 credits.
Powers will be unlocked through dialogue with your companions. Each has 2 powers to give, one generally unlocking fairly early and the other one significantly later. Make sure to talk to them after a mission both on the Normandy and in the various Citadel districts.