Why doesn't the power connector on my thunderbolt display fit in my 13" macbook pro?
Your Thunderbolt monitor has a MagSafe Connector - your MBP has a MagSafe 2 Connector. Apple now supplies a MagSafe to MagSafe 2 converter with the display (check the packaging - the adapter is small). If you are missing the converter, check with Apple - they should give you a replacement.