I just completed Mass Effect 1's main storyline and I'm a bit confused. I'm reading that upon completion of the game, levels 51-60 are unlocked. But I don't see a way for me to resume the character as opposed to starting over.
When you start a new game, in addition to the Default and Custom Shepard options for your character, you should have the option to "Use Existing ID". This will allow you to begin a new game with your existing character (and continue to level up to level 60).
It's a "new game plus" bonus, so the additional levels are unlocked to allow for character build progress in a second playthrough.