Has anyone used the CI-Merchant library for PayPal?

[eluser]Unknown[/eluser]
I'm having the same problems.
After getting the white screen I added

Code:

ini_set('display_errors',1);

and saw that it needed the language helper, so I added that with

Code:

$this->load->helper('language');

.
Now I'm getting an error: Missing argument 1 for Merchant::purchase_return().
I'm not even sure if I should be calling this as the comment mentions hosted payment gateways only?