If you are trying to boot from the OSX DVD and the disk utility in the OSX install utility is not recognizing the 500gb hard drive inside the external enclosure, it is most likely because the only drives that will show up in the install utility will be the internal drive and an external FIREWIRE drive. Try hooking up the enclosure to the macbook using a firewire cable. I might be wrong on this but I have experienced the same on other macs.
Another thing you could do is to go ahead and install OSX on the SSD and once you are finished with the install then you should see the 500GB hard drive on your desktop or in finder or disk utility. I think this would be a better choice since you want OSX to run from the SSD anyways to take advantage of the mega fast speed and just use the 500GB as external storage.
If you still don't see the hard drive then either something is up with the enclosure or the hard drive. Try another hard drive in the enclosure to check if it recognizes that one.
I hope this helps, good luck!!