We use an algorithm similar to Microsoft's TrueSkill, as this is generally agreed to be better than ELO for team games.
Your MMR rating shows your skill level compared to other players. As this reflects skill, playing more games will not raise your MMR by itself. If you want to raise your MMR, you'll need to get better :)
After each game, your MMR is updated based on the result of the game, as well as the current average MMR of each team.
If you were matched against a team with MMR higher than your team, and you win, your MMR will be raised more than normal. Similarly, if you are matched against a team of lesser skill, and lose to them, your MMR will drop more than normal.
With MMR fluctuating like this, after playing for awhile, you should end up with a pretty accurate MMR, and a win rate pretty close to 50%.