I don't have fresh marjoram. Can I use dried? Or another fresh herb?

Oregano is a close relative of marjoram and it's one of the few herbs that tastes better dried rather than fresh.

As Nancy explains, yes, you can substitute dried herbs for fresh. The rule of thumb I know is to use 1 part dried herbs in exchange for 3 parts fresh herbs. Checking first that the dried herbs aren't old and still have good aroma and flavor when you pinch, smell and taste them.

Josie, I wouldn't worry too much about marjoram, fresh or dried. It gets a little lost with the horseradish in any case. If you want to substitute another herb, why not thyme, fresh or dried, to offset the pungent horseradish flavor. In general, it's a good idea not to feel put in a box by recipes. They're meant on the whole to be suggestions rather than formulas--the more you cook, the more you'll discover that there are no hard and fast rules, except perhaps for pastry.