As I'm passing by I'll fill in for Shawn Grim here in referring you to one of his posts which answers some questions about this:
http://www.learningstrategies.com/forum/...=true#Post50394I think there is a lot of other helpful stuff about breathing that can be found through the Search the Forum link above.
And, yes, it is done during the active exercises and the meditations and is definitely an important part of the practice, but I would suggest not to "focus" on it too much and maybe let yourself relax into it comfortably over time.
It can be helpful to remember/focus on Chunyi's three secrets of qi gong:
Going into the emptiness
Keeping it simple!
Using your consciousness/awareness.
I think as long as you remember these in your practise, you're following the right track.
Happy breathing!
Theo T