Is it .xls or .xlsx? I have not tried JMP 11 yet, but I would be surprised if the ability to directly open "old" excel files is gone. If your file is a .xlsx try to save it as .xls (Excel 97-2004 format).
To open an .xlsx file you probably need an ODBC-driver. Regarding ODBC drivers for Mac I think that it's the one for Access that has the best chance to work with Excel. Years ago I tried their trial version and managed to retrieve data from an Access table. It's read only though.