I raised this issue directly with Mathworks support. Here is the reply I received:
The ability to read XLSX files is not available on the Mac OS. On the Mac OS, MATLAB does support a 'BASIC' mode that works with XLS files that are saved in the "Excel 97-2003" format. Since the connection to Excel is based on the Microsoft specific COM technology, all functionalilites of Excel are not available on the Mac OS.
In order to work around this issue, please use the following steps:  Copy the data into a new Excel Workbook and saved it as a 'Excel 97-2003' with a '.xls' file extension (I named this file 'test.xls').  Using XLSREAD on Mac OS, read the data from the XLS file mentioned in step 1.
Alternatively, you could save the data as a Comma Separated Values (CSV) file and read it using CSVREAD. Please note that CSVREAD works only for numeric values. For more documentation on CSVREAD, please use the following command at the MATLAB command line.