Once you've created a free business account on PayPal and requested your access data, you can create a PayPal Sandbox Account to make test payments with one of the Payrexx tools.
Log in at the PayPal developer website. Create a business account (-facilitator@) as well as a personal account (-buyer@) by clicking Sandbox -> Accounts -> Create Account:
At Sandbox -> Accouns -> facilitator profile -> API Credentials, you can find your username, your password and the signature:
Go to your Payrexx Backend and navigate to settings -> payment providers -> paypal -> edit:
Once you are in the settings of PayPal, set a check mark at "Use Express Checkout". Insert the API Credentials (username, password and signature) there. Don't forget to choose the mode "Sandbox":
Afterwards, you can make a test payment on your payrexx Shop with PayPal. Use your personal e-mail adress (-buyer@) with your password when making the payment.
After that, you should be redirected to Payrexx and see the notification, that the payment has been processed sucessfully.
On the PayPal Developer Website, you can find the confirmation of the transaction at Sandbox -> Notifications.