I understand that some time has passed since you asked this question, however I ran into this problem. In my case (using HomeBrew) I just installed ImageMagick and the PHP module using:
brew install imagemagick brew install php55-imagick apachectl restart
It is important to restart APACHE after installation.
As for your question (MAMP), there are many instructions on the Internet for installing ImageMagick using MAMP, see How to install Image Magick for MAMP (Pro) on Mountain Lion (10.8.2)
As one comment says, ImageMagick comes with MAMP, you just need to enable it.
Hope this helps someone :)
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