Either update your PC Excel, or save your Excel files in .xls format: the same will apply to Word docs and Powerpoint. Office:Mac will default to the .xlsx, .docx and .pptx file format.
Go to Preferences for each programme, 'Save' tab, then change the preferred "Save .... files as" to the 97-2004 option.
Resave your logbook file and it should now be in .xls format.