Personally, I dont like to prime a keg. I've done it a couple of times and just didnt care for it. If you leave the gas on for a few days, you'll get it carbonated just fine. If you want to speed it up, crank your regulator up to 25-30 psi and you'll be all carb'd up in even less time.
Now, if you want to get all scientific and precise about it, you'll want to take a look at the carbonation cart posted here:http://www.kegerators.com/carbonation-table.php
If, after taking one of these routes, your beer still isnt carbonated, you probably have a leak in your keg gaskets, line or connections. Crank up the gas and spray the hell out of everything with star san. If theres a leak, you'll see bubbles!