This doc outlines how to install the IonCube loader on MAMP. These instructions were written with MAMP Pro 1.9.6.1, but this should be generally the same for any version of MAMP.
Download the IonCube loader from the download page. You probably want 'OS X (x86)', but if you have one of the newest 64-bit versions of MAMP, you'll want OS X (x86-64)