I know how to find the wifi sync option in Settings, but the Sync Now button is greyed out, and below the button, it says Sync will resume when [Computer's name] is available. I don't know if this has to do with having a windows computer. My iTunes is up to date and open as I try to start a wifi sync. I don't see how my Windows 7 is not "available".
I believe iTunes must be running on you computer in order to sync. If iTunes is running on your computer and you still get this message, try syncing your iPhone using the USB cable, then it should sync via WiFi. I have to do that once in a while to get it to work.