Castrol Super Clean should work well.

 

racer x is right about the Castrol superclean I had the same problem on my DS after using a waxy type chain lube. You should just be able to spray it on the sprocket, swingarm, wait about a minute and the gunk should just spray right off with your water hose try not to get to much on the chain though I did and it didn't hurt it but still you shouldnt take chances. You should be able to get the stuff at any auto parts store.