Today I needed to install ionCube 5.3 to run WHMCS locally and it was surprisingly easy :)

1. Download ionCube from  http://www.ioncube.com/loaders.php
2. Copy the whole ioncube folder to
   

   /Applications/MAMP/bin/php/php5.3/lib/php/extensions/

3. Open /Applications/MAMP/conf/php5.3/php.ini
4. Add this line somewhere around line 1088 
   

   zend_extension="/Applications/MAMP/bin/php/php5.3/lib/php/extensions/
   ioncube/ioncube_loader_dar_5.3.so"

5. Restart Apache.

Done.