error message: need a digital ID

this is a very fundamental question. I’m trying to transition from PGP Corp 8 and have installed GPG. I successfully imported (I think) all of my PGP keys. But when I try to send an encrypted e-mail I get an error message: You do not have a digital ID.

I suspect that my question is too low-level. What can I read to get set up? We’ve been using PGP 8 for more than four years, and it just works. I’m sure when I get GPG set up it will also work, but I’m struggling with the fundamentals.