The ability to use XLSWRITE on a Mac system to create Excel files is not available in MATLAB 7.12 (R2011a). XLSWRITE instead creates CSV (comma-separated value) files when used on a Mac, and provides the same functionality as it does on Linux and UNIX platforms.
The full functionality of XLSWRITE depends on the ability to instantiate Microsoft Excel as a COM server. COM is a technology developed for Windows platforms and is not available for the Mac. Thus XLSWRITE as well as XLSREAD have limited functionality on the Mac platform and work in basic mode.
Support for XLSREAD on Mac OS X may be considered for a future release of MATLAB. Unfortunately, there are no workarounds for the issue at this time.