Changing the default open-with "application" on a nautilus desktop?

[kazaam <kazaam oleco net>]

Hi,
I wanna use PCMan for openening folders on my desktop. I can do right-click on a folder and choose "open with PCmanfm". This works without any problems! But I don't want to do everytime a right-click and choose this option. So when I now do right-click on a folder and go to the settings to make PCman to the standard it doesn't work. I can't switch there from the nautilus "open with" to PCman ? Why is nautilus forcing me to do so? 

greets
-- 
kazaam <kazaam oleco net>