I believe you're referring to PayPal Credit (formerly known as Bill Me Later)..??
In order for that to work that way you need to be sure you've specifically selected the PayPal Credit option as your funding source during checkout. If you don't do that it will use whatever your default funding source is as opposed to charging the amount to a PayPal Credit account, which is when you would get the 6 months deal.
When you do specify PayPal Credit it will take you through an application (the first time it's used) and then you would see a separate balance for that (just like a credit card) in your PayPal account.
Angell EYE - www.angelleye.com PayPal Partner and Certified Developer - Kudos are Greatly Appreciated!