Unknown Class **** in Interface Builder file

I did some research and found out that

for my case

I just need to right click to complaining file in the Project Navigator.

Choose View/Utilities/Show File Inspector. And for some reason the Target Membership was not selected.

That's why the linker says that it doesn't know the existence of the class.

I just ticked to choose it and everything works like a charm.

If you go to Build Phases within your targets you will see Compile Sources. Make sure each of the m files you need are listed. If not, just add it here and the warning should go away.

Right click on Storyboard --> Open As --> Source Code

Find your unknown class name in the storyboard source file. It may be because you have accidentally assigned a custom class name for one of your UIView subclass'.

It turns out the bits on the iPhone simulator were out of sync with XCode. I did a 'Reset Content and Settings' and everything works :) Who knew?