Excel cannot open file "xyz.xlsm"...

I have and old .xls file with macros that opens and functions just fine in Excel 2010.  After saving the file in the newer format .xlsm I get the error "Excel cannot open the file "xyz.xlsm" because the file format or file extension is not valid".

What gives?  The original file works fine.  Is there some trick to saving a file in the newer format?

Thanks.

Peter Ekstrom