Thoroughly clean the rifle, including removal and cleaning of the breech plug. Make sure to get acquanted with that ramrod. Reinstall the breech plug to just snug and make sure you use a good anti-sieze lube on the threads. Your load options are extremely limited so I hope two pellets and a powerbelt do the trick. If not, try some loose powder at about 70 to 80 grains and a good quality 300 grains XTP sabot bullet or (better IMO) a quality conical such as the PRBullet Ultimate 1 in about the 400 grain size.

You will benefit a great deal from going to http://www.hr1871.com/talk/index.htm - enter the talk forums and read the Huntsman section in particular. Lots of good tips and tricks there.

Oh, and read your owner's manual.

Welcome to muzzleloading!