Ooops, I missed this POST. Anyway answering for the records.
Try first just adding a Loader (or MC, don't know your AS version) as background to see what happens. Also you should use a NetConnection call (or SharedObject property) to allow know other users which document to load. I mean you need at least a medium expertise with FMS and AS.
A working example on flashcomguru.com, they sell a very complete whiteboard