How can I fix a leaking expansion tank?
Remove the tank, re-dope the pipe or wrap it in teflon tape, and reconnect the tank. You'll want to shut the water off first as gregmac aptly suggested; ideally there is a shutoff valve on both sides of the tank, otherwise the water in your house will drain out at low pressure.  If you only have one, you'll want to relieve the downstream pipes of water through the faucet closest to the tank and then be prepared to catch any excess with a bucket(s).