Just saw you were running lollipop. Give the above awnser a shot.
You need to reinstall a Gapps package. That isn't the pico or nano
Step 1: Here is a link you might want
Download gapps kitkat
Step 2: Reboot your phone into the custom recovery (which should already be installed).
Step 3: Press install from sdcard and locate the zip.
Step 4: Wait for it to install and then reboot the device. And once on it should appear...
If you can't overwrite the Gapps via this method the easiest way to get face unlock would be to get the apk somehow...