'*.mswmm' is the WMM project file, not the end result. If you're ready to save your work as a finalised video; from the Task Pane on the left, select Save to my computer (3. Finish Movie) - you will be presented with various size/quality options, some save as *.WMV others as *.AVI.
-------------------- TFC 'Going to trial with a lawyer who considers your whole life-style a Crime in Progress is not a happy prospect.' - Hunter S Thompson (1937-2005)