Hello Kevin,
As you will soon discover (if not already done) the version you have downloaded have a lot of problems and bugs !
Follow Juan advice and download the version from https://github.com/PukunuiAustralia/moodle-mod_certificate ASAP.
As you are running Moodle 2.0.5+ you need the version from the MOODLE_20_STABLE branch, and not from the 2 other branches (master and MOODLE_21_STABLE) be carefull to download the right version.