I have searched the net and the consensus seems to be to add ${SDK_DIR}/usr/lib/libxml2 to the 'Header Search Paths'. I have done this and I can see in the project inspector that it expands the to correct full path name. I'm still getting errors that it can't find libxml/tree.h, etc. I have also tried using just /usr/lib/libxml2. That didn't work either. In both cases when I look at the command line generated by Xcode I don't see a -I with my Header Search Paths.
Try adding $SDK_DIR/usr/include/libxml2 to Header Search Paths.