Interface Builder won't open files

I'm using Adium as instant messenger, I want to try the unified look on the Adium windows, someone on the Adium forums once explained how to do it.
He told me to quit Adium, open the recources folder and open the Contactlistwindow.ni with Interface Builder. Now that's not the important part of my story, my problem is that, when I control-click on that .nib file and open it with IB, it says that it can't open the file. I get the message: 'The document ContactListWindow.nib could not be opened.' And that's it.
I don't know what's the cause of this problem, can anyone help me out?

I have not attempted what you are trying to do but it seems a bit dangerous to me modifying the .nib files that compose the user interface of Adium.

It could well be the programmer of Adium has locked down the .nib files to stop people editing them.

It could also be a dependency.

Maybe copy the .nib file, outside of the application package and drop it on the desktop, load Interface Builder and load the nib file and see if it will co-operate that way. If it does save the changes (you need to command + shift + i to get the inspector in IB) you make, and save the nib file back inside the Adium application package.

Vicky