Purpose of PayPal Payments plugin is allow online payments for user registration in “directory” type Themes like CityGuide and Directory+.
After plugin purchase you can install plugin in a standard WordPress way.
In Plugins options screen click on Add New and Upload Plugin button. Next screen will ask you upload zip file with plugin. Select zip file with plugin downloaded from your profile dashboard.
After plugin activation you can turn on/off Paypal button in Payments Theme Options and new options for PayPal will appear: