Please follow the link, you need to login your paypal account: https://developer.paypal.com/developer/applications/
Then please push Create App.
Then enter your application’s name and push Create App
Switch your application in Live mode (it is important) and copy Client ID
Go back to iBuildApp web-site, push Shopping Cart Options, choose Cart with check out. You may set up API signature credentials (Client ID) in Manage Payment Option.
Then choose type of your account (Business or Premier), Paste your Client ID, Enter your email and press Start accepting PayPal.