Can't figure out to change file association in XP.

I am having file association difficulties with my flightplans under win XP. See, the problem is that there is a software of my wifes entitles Mastercook by seirra. It automatically claims any file created with a .pln extension as a mastercook meal PLAN. I right click and choose to default open these files with notepad but it doesn't allow me to change the frame program associated with the file. It always stays listed as a Mastercook mealplan. How can I change this???? Its causing my flightplanner not to list anything with-in the sim.