I think you must
either change a Windows setting:
....Open any map with Explorer
....Select: Tools (or Extra)/ Options / Filetypes
....Select "AVI" and change the present setting.
or in BSPlayer's "Options"/"Preferences"/"General" (=most left tab)
....in the right hand "column" choose the extentions to be "associated" with BSPlayer
....and then press "Register ..."