Download the Facebook App
If you have not installed the Facebook app to your device yet, check out how to get your download from the App Store in this brief tutorial by Sam Costello, Guide to iPhone/iPod at About.com.
How to Access the Facebook App on iPhone, iPod
To start using Facebook Chat on your iPhone or iPod, locate the Facebook app icon on your device and tap it. The Facebook icon appears as a blue square with the iconic lower-case "F" on its face. Once launched, your screen will appear as illustrated above.
Click the "Chat" menu option to continue using Facebook Chat.