I had pointed mine into the /usr/... folder (and changed the port #'s to standard) and had the same error. setting the folder path back to the MAMP default solved my problem (left the ports at 80 and 3306, tho).
hope that solves your issue.
I don't know how to change the default folder for MAMP, so don't think I did that. I have those ports as you spec too.