If you are trying to connect Paypal Express Checkout to OpenWater for collecting payments, you will need to acquire your Paypal API Username, API Password, and Signature. These items will be entered into your OpenWater payment configuration like so:
Your first step is to log into your Paypal dashboard, click Tools in the top navigation and then All Tools.
In the left side bar of the tools page, click Integrate PayPal and then click open on API credentials.
Scroll down to NVP/SOAP API integration (classic) and click Manage API credentials. It may send you a text message code for extra security.
If you have not yet acquired credentials in the past, it will ask you to Agree and Submit. Leave the box checked for Request API signature.
Click Show on each of the three items and copy and paste them back into OpenWater.
There is a fourth field, Reference Coding in OpenWater. You can put "OpenWater" or "OW" in this field. It is a marker that gets passed to paypal to let you know that transaction came from OpenWater and you can use this to refer back to payments for reconciliation.
If you have any issues acquiring the above credentials, you may need to contact PayPal support as this is the extent to which we are able to provide guidance on your connection.