Roberto,
You solved my problem for me, Thanks!!!! But, there is an easier way.
After you have created your course, add a final activity naming it however you want for them to print out the certificate. I chose "(Name of Course) Completion Certificate" In the settings, you can qualify the reciept of the certificate by a quiz score or a cummulative score on the whole course. I tried it and it works.
Clint Randolph