NSS: client certificate not found (nickname not specified)

Working with the Paypal API, yes i've checked my config files, yes i've checked username and password.... i'm outa ideas. I'm using the ExpressCheckout API downloaded from and everytime i try call the setExpressCheckout method i get the following error:

NSS: client certificate not found (nickname not specified)

I know it's something to do with an SSL error, how do i go about solving the problem?