There is no such thing as "float: center;"
http://www.w3schools.com/cssref/pr_class_float.asp
CSS is quite theme/site specific so for CSS help, you need to post a link to your site.
Also, this thread is marked resolved, so not a good place to ask a new question - please start your own thread -
http://wordpress.org/support/plugin/contact-form-plugin#postform