Make sure that you're installing the files for the add in module in the right folder. If you changed the admin directory to another name during the original install, you need to make sure that you're using the renamed directory and not admin.

I had forgotten that I had renamed the admin directory and I was getting this error because I was loading the files in the wrong directory. I was trying to install Easy Populate but it's probably the same problem with the module you're trying to install.