I want to teach my Seismitoad Earthquake but I can't find the TM.
It's at the end of Relic Castle. Go to where you met Ghetsis first, and go up the stairs. Keep going up, and you'll find Earthquake!
It's inside Relic Castle and it's actually kinda confusing to reach (I think). And the Earthquake TM room is only accessible in the post-game after beating N and the Elite Four.
This guide has step-by-step screenshots showing you exactly how to reach the TM.
Here's a screenshot of the TM location, taken from the guide above:
