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It's probably a silly question, but I can't figure it out for the life of me. How do I add basic lands to a deck I'm constructing in deck editor?

It's the same as adding any other card. The only thing I can think of that you might be doing wrong is filtering by color and expecting to see the basic land in there. Basic lands are colorless, and thus if you are filtering only red cards, a Mountain does not appear.

Figured it out, that's exactly what I was doing. Thank you!