I assume you installed by booting off a bootable sdcard.
1. Make sure when you boot off your sdcard that it says version 18.104.22.168 (or check the file you used to flash it). If it's not that version it needs to be updated since it will error out when formatting data, which we need to do when changing roms.
2. Download the latest gApps
. These are from my guide
(3rd section). Copy them to your sdcard.
3. Boot off the card and flash the gApps using my guide above, BUT skip the "format system" and flash CM7 part, since they aren't necessary (but won't hurt anything). Make sure you format data tho.
Let me know how that works for you.