with the new OSX 10.8 and earlier OSX... (MAC) there may be some problems with the Java VM.
So the installer window don't work correctly.
Please try this way:
There is a file 1.6.0....jdk into the folder System/Library/Java/Java Virtual Machines/ . Please delete this file. But make a backup of the file before and save it to another location.
Now run the installer again and when the MAC ask whether it should install the required Java VM, then answer with "yes".