ThemeKey UI: Validation of 'Selectable Themes' seems to be broken

Hey there, I just installed Themekey and noticed that in the Themekey UI settings there is an option to set a specific theme at the time of a node's creation. I enabled this and then moved to create a node, but didn't see any specific dialogue that would allow me when creating a specific node (I have it enabled as an option in the themekey settings) to set a specific theme.

Basically, I have a site that will have 2 content branches each with their own theme.

I am enabling signups to events, but depending on the type of signup, I would want one of two content brach themes to be used.

Instead of creating content, and then going back into the rules listing and setting it each time, I thought this feature would be great, but for some reason I can't get it to work or display.

I assume I am missing something very basic.

Note: the only theme related dialogue I see displays a radio button saying something along the lines of "do not allow the theme to be switched"

I have more than one theme enabled, and can manually set themes via rules, but this theme-on-the-fly doesn't seem to work.