Get your iPhone Connected via WIFI
go to http://www.unlockit.co.nz (You will see an iPhone Formatted Website)
Choose Create APN
Choose United States
Choose Straight talk
Choose Create APN below the Carrier
This will set the APN Data Settings for Straighttalk.
This does not jailbreak or hack your iPhone in anyway and the website is used successfully by tens of thousands of people users per day all over the world. It uses the Same method as the iPhone Configuration Utility but some people find the iCU quite hard to use (or they don't know the settings/where to put the settings)