Segmentation greyed out

Hi!
I'm composing an invitation e-mail as part of a larger campaign, and I'm interested in doing A/B testing depending on demographics. 

Under Segment By, I attempt to move the Subject to the Dynamic header, then I get the popup that says Segment By. I try to choose an option (such as location) and all the options are greyed out.

Each person in our database has a first name, last name, e-mail, and so on. I should be able to segment by any of these. Any idea why it's not working?