In the Power store settings, set the Payment gateway option to PayPal standard, then enter your pay[pal account email address.
In your payapl account, enable IPN and set the IP Post back address to the address in the settings panel next to "IPN Page"
PayPal standard is a free service with paypal, transactions are not performed in real time, In other words, the checkout success page only means that paypal has excepted to process the transaction, it can take up to a day for the transaction to be processed with the merchant bank.
Paypal standard is also a remote checkout method where the customer leaves your site to set up the transaction.
payPal Payments Pro is a local checkout method, the customer never leaves your site, they enter credit card info directly on your site, the transaction is also processed in real time.