According to Trek, it's a Shimano RM-30 hub you have.
Don't remove the dust cap. Just remove the left side lock nut, any spacers or washers, cone, and then the axle and right side cone will come out. Catch all the bearings on a shop rag, and then clean the inside of the hub. Put in new bearings with plenty of grease then reassemble and adjust. It'll last for years if you do this every year or two and you keep water out.
More detail:
http://www.parktool.com/blog/repair-...and-adjustment
Your hub probably didn't come with a rubber seal. Don't worry about it.