you stumped me. I think there is a critical piece of information we are missing though. I just do not know what that is yet

perhaps something like: you are not importing the entire NGUI package, it really is an older version... I don't know
that method is used all over the place in NGUI so you should see a bunch of errors, yet you do not. that is something...
For instance, in NGUIMenu.cs on line 98...you do not get an error there yet it calls the same function.
If I were you I would try changing the offending line of code until it worked, to figure out what is going wrong. try right clicking on it select "go to declaration", etc. I think if you fiddle with that it will become apparent what went wrong....