Okay found the solution thanks to Wessel Bosman:
I have Mendix Business Modeler 5.2.0, Build 1371
Had similar error: Unable to render loginsystemwidgetLoginForm_0 widget: Cannot read property 'login' of undefined
Seems the solution was easy, when your installing the Login Form to be sure it is version 2.0, click on the All Versions tab and click the download button below the padlock icon (should be right at the top of the list of versions).
To view a print screen: https://docs.google.com/file/d/0B4OdJxejllfEd1o1R0x6TGJvNlE
Make sure you install Version 2.0, when i did the login did work like a charm.
Regards and hope this can help. Gabriel