I am having the same problem as a lot of the people on this forum with the exposeMask jquery function that is packaged with your plugin. It is interesting that so many themes are supposedly interfering with the way this plugin functions.
Simply, the registration screen comes up but is hidden behind a white mask. It should obviously be in front of it. I went to the login-with-ajax.js file and played with the function a bit (changing the value of z-index) and had limited success. I succeeded in getting rid of the white mask part but the z-index is still sitting behind my custom nivoslider - no matter what I seem to do.
Fully desperate for an answer here......