Control+8 is still not working, it stop the video and take me to the start of it.
Set AE so that it will do a looping preview.
Highlight the layer you want to mark.
Begin the RAM Preview by hitting the zero key on the numeric keypad.
Watch the preview a few times until you know where you want to add user marks.
Add user marks by hitting the Asterisk (*) key on the numeric keypad.
I don't have a numeric keypad on my MacBook but I pluged my old keyboard wich have a numeric keypad and it's now working, thank you so much! (I was trying to solve the problem till one year)