shell
sudo su
cp /etc/chrome_dev.conf /usr/local/
mount --bind /usr/local/chrome_dev.conf /etc/chrome_dev.conf
echo "--arc-availability=officially-supported " >> /etc/chrome_dev.conf
restart ui
exit

Once you have logged back in, you should see Play Store available from the main menu.
If you turn off your Chromebook (complete power off opposed to sleep), you will need to re-enter some of the code above from terminal (Ctrl & Alt & press T):

shell
sudo su
mount --bind /usr/local/chrome_dev.conf /etc/chrome_dev.conf
restart ui
exit