Does the file exist directly below the root of your site in /bin/sts.FormWizard.Business.dll ?
The installation requires installing the "contents" from the wwwroot folder in the dotnet40 folder into the root of the site. This means that you don't copy the dotnet4 folder nor the wwwroot folder, just the contents of the wwwroot folder, and you paste or upload that directly into the root of your site.
If the files does exist, then I would try uploading it again in case it got corrupted during upload.
Just for sanity I tried a clean installation of mojoPortal 126.96.36.199 and Form Wizard Pro here and it all worked fine.
Hope it helps,