After spending hours searching the ENTIRE internet for an answer, and spending hours changing bits of the code, i found out that the answer to this is pretty simple
In the simpleviewer source code, in the actions on the 2nd frame, there's an actionscript command that says:
"Stage.align = "TL"; "
This aligns the simpleviewer (and any flash file you embed it into) to theTop Left corner. Just delete this line and the file you've embedded it in should be centered in full screen