First, go to the Admin section and click on 'Payment Modules'. Install and Enable the PayPal module. Please enter your PayPal address carefully. Some other settings will be recommended to you, so it is best to enter the recommended settings.
To get the Instant Payment Notifications to work, ensure that IPN is enabled for your PayPal account.
Log into your PayPal account and click on 'Profile' -> 'Instant Payment Notification Preferences' and hit Edit.
Enter the IPN URL of your website that was entered in step 1. (If you already have a IPN URL entered in there, don't worry about it, the Million Dollar Script will pass the URL to PayPal and PayPal will know what to do anyway)
It is possible to test PayPal using the PayPal sandbox server. You will need to select the sandbox server from the drop-down list, and you will also need to register for a PayPal developer account.