In Admin CP in Payment systems (http://www.your-site.com/admin/pay.php) set checkbox Active in  PayPal, near “Bussines” enter your PayPal email, near Currency code enter the currency. Main here are USD, EUR, the codes of other currencies are available in PayPal help.

In your PayPal account go to Profile, there go to Instant Payment Notification Prefences. There you set Instant Payment Notification to On and enter Instant Payment Notification URL http://www.your-site.com/_pay/paypal/after.php. After this press SAVE.

After this click Back to Profile Summary and go to Website Payment Preferences.

There you set Auto Return to  On and Return URL to http://www.your-site.com/_pay/paypal/after.php

Press SAVE.