How can I activate spell checking in Excel in Office for Mac 2011? I would like to have spell checking just the way it is in Word or system wide in OS X.
You cannot. Office:mac does not use the system wide OS X spelling service. Instead it uses its own set of proofing tools, like the Windows version does. This gives you “as you type” spell checking in Word because Microsoft supports it there, but as to Excel, Microsoft states: