I have a RB751 and am mostly very happy with the default QOS management. I have a Dreambox which needs to send receive 3-4 packets (about 350bytes) every 8 seconds, therefore I'd like to prioritise it as I have some teenagers who are heavy torrent and Youtube users. Is there a simple way I can prioritise traffic to and from this external address through the router.

I'm not good at English, but I can tell you have to check connections and Firewall packages - Mangle and using HTB will prioritize, guarantee bandwidth for a destination or application. See the Wiki if you help.
Follow the link

http://wiki.mikrotik.com/wiki/Manual:HTB

Is there a simple way I can do it without setting max limits etc.

I'm coming back to this, I still have not implemented it. I'd simply like to priorities traffic to and from a device on my lan. Is there a simple method? It is always to and from the same external IP and always on the same port.