Here are the instrutions for Payal.

1) Login to our system
2) Go to the label configuration page
3) Under paypal payments - make sure paypal is enabled.
4) Log into paypal
5) go to Profile
6) Click on API Access
7) Click on Request API Credentials
8) Click on Signature
9) Submit and view the credentials.
10) Go to our system
11) Input API user name
12) Input API Password
13) Input Signature
14) Click submit

Make sure if you copy and paste, there is no white space at the end. Let me know how it goes.