Your question is too vague. What do you mean by "doesn't work"? There shouldn't be anything wrong with the assignment (verify with an NSLog() + NSStringFromRect() before and after).
My apologies about the vagueness. After reviewing the NSLog(NSStringFromRect(myNSRect)); messages I determined that I was, indeed, moving the NSRect, which then lead me to realize that my problem was resetting the frame of my other element (the frame of which I used for the basis of my NSRect), which I wasn't doing.
Code:
[self setFrame: myNSRect]
That was the piece I was missing. Thanks for the heads-up on the NSStringFromRect() method...and your patience! (If you couldn't tell, I'm still a newbie at all of this).