Pro Site's Instruction to PayPal Signature Not Working

Hello.

I am trying to configure Pro Sites with paypal. In teh PayPal API Credentials section, there is a link with instructions to set that up in my paypal account. That link is not wokring for me.

I think the link is: https://www.x.com/developers/paypal/documentation-tools/express-checkout/integration-guide/ECAPICredentials

Thank you!