PayPal's Payflow Pro Credentials should be entered in the Chargent Gateway record in Salesforce as follows:
1. Gateway Name - Any name you choose, for example "PayPal Payflow Pro Test"
2. Merchant ID - Merchant Login
3. Merchant Security Key - Your PayPal Payflow Pro password
4. Merchant Reference- Partner name (for the test account it will be "PayPal", but for production if you signed up through a Bank rather than directly with PayPal it may be different)