Hello crzyanimemom !
I guess you mean "how do I register for the ball" ?
Normally, you only have to click on "Registration" and "Choose" (under the species you want to register).
The pop-up only show you the creatures who are available with all criteria : one background and at least three accessories.
For me the pop-up take a few seconds before open up.
Are you sure your creatures are prepared with a background and three accessories or more ?