After the Purchase Order is created, you need to receive the items so they will be included in inventory.
Click on Purchasing from the main menu (shopping cart).
Click on the Edit icon next to the PO you want to receive against.
Click on the down arrow on the green Receive button. You have an option to Receive or Reject the PO.
When you click on Receive, you can fill in the receiving quantities individually, or you can select Fill PO Balances to automatically populate with the remaining quantities.