Try editing your eclipse.ini file and add the following at the top
-vm
/Library/Java/JavaVirtualMachines/jdk1.7.0_09.jdk/Contents/Home
Of course the path may be slightly different, looks like I have an older version...
I'm not sure if it will add itself automatically. If not go into
Preferences --> Java --> Installed JREs
Click Add and follow the instructions there to add it