For the last month I been trying to install the gread add-on Simple Forms with no success. The guys at LiquidPro helped me and finally got it solved. Now the add-on is installed the way it should be. It turns out that for the Simple Forms installation to work the xf_user table needs to be set to storage engine InnoDB since the their tables uses foreign keys. In my default installation fo XenForo the table was set to MyISAM.
After running the following code the installation worked:
ALTER TABLE `xf_user` ENGINE = 'InnoDB';