Buying POW is extremely easy. Just enter an amount in the Quick Buy/Sell & Transfer widget (make sure that BUY is chosen on the right) and click on Transfer Now. As long as you have sufficient EOS, RAM, and CPU, the transaction will be processed automatically, and in a couple of seconds you'll have your POW.
If you see an error message saying your balance is overdrawn, it means you don't have enough EOS to buy the desired amount of POW.
In this case, load more EOS into your account using this feature:
If you get an error message saying you don't have enough resources (CPU), refer to this tutorial.
If you want to sell your POW for EOS, use the same feature in the dApp but select SELL instead of BUY.
To transfer your POW to another user, select the transaction type Transfer in the Quick Buy/Sell & Transfer widget, enter the amount and the recipient's EOS account name: