why can't Xcode find this header file?

Hi I'm getting an xcode "no such file or directory" for:

#import "Three20Core.h"

I also note when I'm typing in the #import statement it recognises, and helps autocomplete, the "Three20Core.h" file, however when I compile I get the error?

This is the main library file header for the Three20 library (from Facebook). So I've got the three20 directory at the same level as my app directory. In the Xcode application target build settings:

1. Header Search Paths - "$(BUILT_PRODUCTS_DIR)/../../../three20" and recursive, and
2. User Header Search Paths - same as above.

Any fault finding advice?