You're probably using some kind of turbo sim unlocking sim.
Some of these take use of airplane mode to make them sync with iTunes.
To jailbreak your phone, you don't need to sync with iTunes.
Download the PwnageTool and do as they say. iTunes will recognize your phone at the end of the process and you'll have to load the custom ipsw through iTunes. But that's all explained in the PwnageTool app.
I think you got two things mixed up.