With PCQ you shoul make use of mangle rules. Checkout this:
http://forum.mikrotik.com/t/i-need-help-as-soon-as-possible/1810/12
But, instead of PCQ yuc can just setup 2 different profiles with different speeds.
Then when create new pppoe user, just assigm him to neeeded profile. Dynamic simple queeue with apropriate setting for that user will be used when he connect.
Regards