Problem with installation

I installed your plug-in but when I clicked on Activate I got an error that the plug-in had an invalid header. I attempted to install again (for lack of a better thing to do) and the install failed because the destination folder already exists. I don’t know how to proceed. I went through the list of plug-ins and this one is not there so I can’t find it to delete it. How can I activate this plug-in? I appreciate some guidance at this point.

http://wordpress.org/extend/plugins/visual-form-builder/