I can advance the video frame by frame with Cmd Num + but when I want to go back with Cmd Num - as given by the manual, it changes the score size instead and doesn’t decrease the frames. Any advice will be greatly appreciated as I am on the steep learning curve right now. 
Lee
P.S. Mac commands
You could also try Command-F7 and Command-F9, which should also work.
Hi Daniel, neither one of those commands work. I have not changed any default key commands in my preferences, so I am a bit stumped.
Lee
Which keyboard language and which language in Dorico are you using?
English and it is a standard American bought iMac from a local dealer here in California. I use this iMac very successfully with Cubase.
Lee
After digging through the Dorico preferences, step by step, I found that in Keyboard Commands, under View, Under Zoom out, the same command Cmd Num- is there. I removed it and now the Cmd Num- works for moving back frame by frame. How that command got there I don’t know. Perhaps on installation as this is a relatively new install.
Lee