You can find out what version of Zen Cart you are running by going directly into your database using a tool such as phpMyAdmin and looking at the project_version_major and project_version_minor fields in the project_version table.
If you are running 1.3.6 then this would further explain your difficulties as the mod was written for a later version of Zen Cart (1.3.7) and messes about somewhat with core code.
I would recommend anybody else reading this thread not to go anywhere near this mod. It went through about 25 releases in quick success to fix bugs, yet when I skim through it, there are still some elementary errors in how the files are structured and it is certainly incompatible with the current version of Zen Cart (1.3.8).
To repair this would take a high degree of skill and may even not be possible. So I'm going to recommend something that is rarely necessary, but in this case seems like the route most likely to succeed, and that's to go back the back-up that hopefully you took prior to installing it.