Paypal in CodeIgniter
Actually, I just went through this. One thing you MUST be aware of is that PayPal (PP) is moving their "new clients over to the new "payflowpro" api. I spent a month successfully developing a cart which made use of the older paypalpro nvp api only to find out that since my client was "new" (she had been using PP for years at tradeshows and such) because she wanted to add online purchasing to her web site - she had to use payflowpro!
No where in the documentation did PayPal steer me to the new api. I chose the nvp solution because there was more documentation on it out there. Do yourself a favor and make sure you are developing for the right api!
Here is a link to a class that worked for me Angell EYE PayPal CodeIgniter Library
Best of all, Andrew Angell is really helpful!
This should be a good resource to get you started. It covers what is required to do payments through paypal, and has some code examples. You can then apply that information to creating your own library, or using the one you previously installed.