Hi kihon, You can’t use “Automatic Redirection” anyway. When you use Auto redirect PayPal do not post the payment data back to the “Thank You” page and without the payment data the plugin won’t display the download links.
I have the same setup as you (I don’t have auto redirection). I did customize the text on the “return button though”. I think mine says “Click here for your downloads”. You can customize this from the “Payment Gateway” settings menu of the plugin.