Dear WPMU, i have a problem with the pop-up in my site.

Dear WPMU, i have a problem with the pop-up in my site.
This pop-up contains a registration form necessary to download a coupon from my site.
When I fill the fields necessary to closing the pop-up, it will not disappear, doesn't matter what I write in the fields.
What can I do?

Plus, I want to cancel the "X" that appears on the upper right of the corner of the pop-up and let became the form filling the only way possible to close the pop-up.
It is possible?