I realise you were after help rather than a solution but I had the same issue running silverstripe version 2.3.1 so thought this might be useful to you.
To get my paypal working I upgraded silverstripe to version 2.3.3 and installed ecommerce 0.6 beta 1 (http://silverstripe.org/assets/Uploads/ecommerce-0.6-beta1.tar.gz and payment 0.1 (http://silverstripe.org/assets/Uploads/payment-0.1-beta1.tar.gz).
Within your _config.php file, add the following lines (change to suit your paypal details):
//E-commerce settings
// Define which payment methods your site uses.
Payment::set_supported_methods(array(
'ChequePayment' => 'Cheque',
'PayPalPayment' => 'Credit/Debit Card'
));
// Check if the site is in 'live' mode, use the live credentials.
// otherwise, use test credentials instead to be careful.
if(Director::isLive()) {
PayPalPayment::set_account_email('youraddress@yourdomain.co.uk');
} else {
PayPalPayment::set_test_mode('yourtestaddress@yourdomain.co.uk');
}
I hope this helps!