Trouble with installing BNW

So, I already have the Civilization 5 standard game and the expansion pack Gods and Kings. I asked the Brave New World expansion pack for XMAS, but my grandfather accidently bought a complete CIV 5 edition (CIV 5 + G&K + BNW) instead of only the BNW expansion pack. So I tried to install the game but it just doesn't add the BNW expansion pack. Uninstalling the game from the pc and reinstalling from the CD didn't change the situation: it reinstalled CIV 5 and G&K but not the BNW expansion pack. The only solution I can think of is creating a brand new steam account and installing the complete edition on this new account. I would lose al achievments etc. but at least i'd have BNW. Any ideas how I can install BNW without having to create a new steam account?