These other guys are dancing around the issue, but I will be a little more direct. I am not sure you can have a load worked up by this weekend if you have never shot a muzzleloader. To do that you will need to spend a minimum of 3-4 hours at the range and that is if your first choice of bullet works out. Muzzleloaders are not like centerfire rifles where you can buy a box of shells, go to the range for an hour, tweek the scope, and you are good to go.
I would ask the person who used the gun last what they used and start there.
As far as actual advice, I would recommend to not exceed 100 gr of powder. My elk load is only 90-95 grains. In other words, 100 gr is plenty to kill a whitetail. Unless you don't have a problem with heavy recoil I would avoid 150 gr loads like the plague.
And as far a powerbelts, you can probably work up a decent load with them, but definately don't use over 100 gr with them. If pushed too hard they are likely to come apart when they hit anything harder than skin at that velocity.