First, you'll need to enable the PayPal selling tools API. Login to https://paypal.com
PayPal classic view: Go to Profile - My Selling Tools and under API Tools, click Update.
PayPal new view: Go to Tools - API Access - Go to API Access
You will then be asked to choose an option, you will want to choose Option 2 that applies to "Pre-integrated shopping carts running on your own server"
Enter that information into Clientexec - Settings - Plugins - Payment Processors - PayPal and scroll to the bottom of the page to enter the API username, password and signature that PayPal provided.