There are already a number of modules written for the Titanium platform, both by Appcelerator themselves and by the community at large. There is even a brand new Open Mobile Marketplace where you can buy (and sell) modules to extend the platform to even newer and greater heights!
You will first need to sign up for the Titanium + Commerce program before you can download and use the PayPal Mobile Payment Library. You can do this for free at http://www.appcelerator.com/products/titaniumcommerce/. Once you have filled in the required form, simply download the ZIP file titled Titanium+Commerce MPL for Paypal Module for iOS to your computer's hard drive.
You must also register your application with PayPal, and on doing so you will be provided with an Application ID that you must reference inside your Titanium project. You can register for an Application ID from http://www.paypal.com. Note that the registration of an Application...