I believe this is a known issue - see this extract from the installation doc:
http://svn.modxcms.com/docs/display/revolution20/Basic+Installation
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MAMP on MacOSX
MAMP (including latest 1.8.4) works fine with MODx Revolution, with one exception. You cannot use eAccelerator as the caching system, as the drivers compiled with MAMP are faulty with regards to PDO and will cause Apache kernel errors. Select the ’xCache’ caching drivers to remedy this.
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The only problem I had with this advice is that it doesn’t tell you how or where to select the xCache caching drivers, so for the benefit of others that have the same problem here is where it is if you are running mamp pro:
click on the server button top left of the mamp pro configurator screen
then php - middle of top bar
cache is a dropdown selection in the box bottom left
I have the recollection that I had to select the Zend optimizer to enable the cache dropdown selector - but it’s not exhibiting that behaviour now so maybe that was my memory playing tricks on me
anyway just select xCache there.
I had a clean install with no errors and thus far a rock solid platform running locally on my MacBook with friendly URLs and the only obstacle my own ignorance!