The short version: after you followed Jean's instructions about renaming the folder certificate, got to site administration and click on notifiction. You should get a notice that certificate can be upgraded or installed. Do it.
Then go to a course and add the certificate activity and follow Jean's second suggestion by going to MoodleDocs
FYI: I have downloaded the latest and greatest form Pukunui and it is working with 2.1.1 on my localhost. There are 2 exceptions. Do not ask it to put in a quiz grade and do not ask it to print the end course date . You will either get a fatal error or your student will not be able to get a pdf. The admin since they are not given grades, will get a pdf.
I know that there were issues with both Certificate and some versions of 2.0.x. Other than the above I could not duplicate those 2.0 issues I saw with Certificate in 2.1.1.