I downloaded ETS' PowerPrep II, which gives you access to the two free GRE tests from ETS (and, I must say, is not as nice as Manhattan Prep's).
It took me an hour to figure out why I kept getting a "Java Runtime Environment" needed message on my Mac (with Mac OS X Yosemite), but my loss can be your gain. Here's what GRE Tech Support emailed me with after I called them:
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If you are running a 10.7 or higher OS, please follow these instructions:
You need a different Java to run the POWERPREP.
First, please remove the current version of Java which you can find in System:Library. Please trash this Java folder. To find the Java folder, you can navigate directly to the folder by opening the Finder, select 'Go' from the Menu bar at the top and then select 'Go to Folder' at the bottom of the menu. In the space provided, type in the following:
/System/Library
Then, you will need to download Java from the Apple support pages here at the following website:
MAC 10.7 LION; 10.8 MOUNTAIN LION; 10.9 MAVERICKS; and 10.10 YOSEMITE USERS:
http://support.apple.com/kb/DL1572
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Hope this helps!