To get Google Play Store works, we need to install gapps from recovery mode.
1. Install a Recovery Mode from http://www.clockworkmod.com/rommanager, column 2 or 3.
It is similar to flash ROM, using fastboot, detailed steps: https://sites.google.com/site/teamroyalsginger/guides-under-development/flash-clockwork-recovery-via-fastboot
3. Copy gapps to your phone. Enter recovery mode, choose install zip from sdcard-> install from zips, then choose your gapps to install it.
4. I tried several version of gapps, after reboot, the system always said “Unfortunatelly, XXX.apk stop work” eg. inputMethod, Setupwized…
One reason is the version of gapps is not right (which is not the cause of my problem)
So I went to \system\apps to delete the not work .apks (if you like, you can also delete the apps you do not want)
5. Then go to recover mode, did the 2 wipes (data, catch,) if needed, do other wipe also. Make sure you format the os. The installed Gapps will not disappear, on the contrary, the system default apps you deleted (not work ones) will come back normally.
6. Reboot the system, now it should works fine. And you can enjoy your devices now.
P.S. Before 1 step, you ‘d better root your devices first.
For mount as rw, please check #3 http://androidforums.com/droid-x-all-things-root/130875-system-r-w-r-o-script.html just run the script.