I don't know from Mac, and I never use drop-down lists in Excel, so I'm not the best person to help. But won't an IF function work? "IF(AND(A1="Metal",B1="Aerosol"),"widely recycled, Check or Not recycled",...).

But then, I see something's missing. What do you want that function to select in other cases? And maybe the IF function won't work in conjunction with drop-down lists at all; I wouldn't know about that.