How do you load a Traditions 50 cal bolt action mussel loader?

Your question answers itself, "muzzle loader". You load it from the muzzle.

Actually, that's incorrect. They use the term "muzzle loader" improperly. It's a bolt action black powder rifle. The powder is pre-pressed into slugs that you load into the chamber, along with wadding and a bullet. I would guess you could load it down the muzzle, but it's faster and easier to buy the slugs and load that way.