Where is the conditional setting about grades?
Your site administrator needs to turn it on in Site administration > Advanced features > Enable conditional access.
My old watermark does not work in 2.x certificate
The default 2.1 certificate type code sets the transparency of the watermark image to 10%. Either change the watermark image so it is darker, or adjust the $pdf_SetAlpha setting. See Certificate customizing and the "Adjusting transparency of an image" section.
Where can I download a copy of Certificate
You can download Certificate for Moodle 2.x from the Moodle plugins database.