Integration between eNewsletter and Membership plugins

eNewsletter and Membership both say that they integrate with one another so you can send newsletters to specific membership levels, though they don't have a lot of specifics on that.

After searching around a bit, it sounds like the membership levels are supposed to show up as checkbox options when you click 'send' on a newsletter. I am not seeing this option. I have upgraded to the latest versions, verified that they are otherwise functioning properly, etc.