I was having issues installing this on Mac OS X 10.7.4 (11E53), MATLAB R2011a 64-bit. I have the latest version of Xcode installed at the time (4.3.3).
The exact error I was having can be found at http://pastebin.com/dnjtuq2Q.
In order to solve this, I first changed compile_cpp_files as directed by Tim earlier (changing OpenNiPathInclude and OpenNiPathLib, modifying the mex command to build 64-bit versions).
Then, I needed to change my ~/.matlab/R2011a/mexopts.sh script on line 167 - changing SDKROOT to '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk' instead of the (now outdated) '/Developer' location.