I would suggest trying to reinstall it again. If you get an error on the plugin page, then it probably isn't installing correctly. Trying downloading the plugin directly to the server and copying the unzipped folder directly to the mod folder in your Moodle installation. Once you have copied it then change the permissions on the whole folder to match the permissions on your other mod folders. 644 should work.
If you still have problems, turn debugging on and then post the full error message here.