To start with:
An AS3 version of the manual is available. It is recommended you skim this before posting a particular question. There's also an API Reference for AS3. There's further information on the wiki, if badly arranged.
Please use the manual and the TestBed, between them they should answer most queries.
How to ask a support question?
There's not really much to it, we don't have many strict rules.
- Don't ask questions off the topic of Box2D, as we are not a general AS3 forum. I appreciate there is considerable grey area, so do ask if in doubt.
- Do not PM individual members for advise, unless it is specifically to do with them. You'll get a better response if everyone can see and contribute to your question.
- Clear description of the problem, or desired effect
- Source code of the relevant bits of code, and compiler error messages
- A zipped swf, demonstrating the problem, with instructions on how to reproduce
- Full, compilable from scratch, source
- A modified testbed demonstrating the problem, if you think you've found a bug in Box2DAS3.
Now go and have a quick look at the AS3 FAQ.