First Calibre problem ever...

I just got a new NOOK (lenova10 Nook)
It worked fine at first but Now when I try to add book files via Calibre the usual way IE usb cable from lenova device to computer... instead of finding Calibre the computer defaults to windows media player (Which I never use)
I'm sure I'm missing someething simple but I think I'm following the directions. Calibre is open in cxomputer...Usb connect from lenova nook to computer, right?
It defaults tp win med player, grrr
Can you help me with this, please?