To access Proving grounds play, make sure you are logged in.
On the Menu bar, you will see "LABS" option and when you click on it you should be able to view "Proving Grounds".
Clicking on PLAY will bring you to the list of PG Play machines.
You have 3 hours of PG Play access per day. If you need more access time, you have the option to Subscribe to PG Practice, which will give you unlimited access to both PG Play and Practice machines.