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This may be a silly question as I haven't actually played MTG draft before and wanted to practice it, but I thought that you normally just pick non-land cards and add land after the draft is done. However, in Forge (I'm on the Mac OS X version) it seems you have to pick the lands from the boosters too, which makes it quite difficult to get to 17 lands of the right color. Am I just missing something?

On the right side of the deck editor there is a "Add basic lands" button (next to the "Remove card" buttons). There you'll be able to add as many lands as you want.