I had to do the same thing - the drain kept clogging. I tried the compressed air trick first, which unclogged the drain and allowed about a gallon of water to come out. However, it clogged up again shortly thereafter. I then used a shop vac and rigged up a hose and vacuumed it out. It then clogged again a little while later. After the second vacuuming, it has not reoccurred.
Keep at it, eventually you get all the gunk out of there.