|Try starting in SAFE Mode:|
EDIT: Your using Mac this is for PC, theroy is the same, locations will differ.
Enter: excel /s
If you get an error message, you may have to use the complete path, something like:
C:\Program Files\Microsoft Office\Office\Excel.exe /s
Safe Mode forces Excel to bypass:
All files in any Excel startup folder.
Add-ins: The Addins dialog box still displays the
selected addins, but, the addins are not loaded.
Excel toolbars (Excel.xlb or <username>.xlb): The
original installation default toolbar settings are used.
Excel specific registry information: The original
installation default registry settings are used.
If this solves your problem, then the culprit resides somewhere in the above locations.