The most common problem with MySQL Server not starting is another mysql service running on the same port. To check this
- Quit MAMP PRO.
- Open the Activity Monitor located in your /Applications/Utilities folder.
- Type "mysqld" into the search field on the top right.
- Quit every process you find after your search.
Restart MAMP PRO.
If MySQL still refuses to start check the log file for error messages.