The KRKs are great speakers.
You're probably going to want something to control the volume with though (believe me, the OS X volume control won't do).
I'd recommend a small mixer, Behringer produce some models which are not much bigger than.
A
Xenyx 502 should do the job. Connect the Mac into the 'CD/Tape In' and hit the 'CD/Tape Main' button, and the connect the KRKs to the 'Main Mix' output, run the volume control on the back of each speaker at 50%, use your Mac at it's full output and use the 'Main Mix' control on the Xenyx to control the volume.
Remember, always switch the speakers on
last after everything else in the setup, otherwise you could get loud pops/buzzes etc...