It looks like you have a custom CSS file for this account that is over 900 lines in length. There is something in the custom CSS file that is conflicting with centering the leadbox. We have confirmed that the sign up box can be centered with one of our prebuilt themes located here: http://allprowebtoolsthemes.com
This forum is for providing support for the AllProWebTools built in features and themes. We try whenever possible, but we cannot individually troubleshoot issues caused by customizations.
I am sorry that we cannot be of any further assistance to you with this issue.