Here is where they are supposed to be:
For 64-bit version: C:\Program Files\Microsoft Office\Office14\
For 32-bit version: C:\Program Files (x86)\Microsoft Office\Office14\

If you have a Click-To-Run installation:
64-bit version: C:\Program Files\Microsoft Office 14\ClientX86\Root\Office14\
32-bit version: C:\Program Files (x86)\Microsoft Office 14\ClientX64\Root\Office14\

Elsewise, launch Excel then open Task Manager and in Processes tab (or Details tab) look in Excel's "Image Path Name" column.