You really need to tell us how you tried to sync them, and how it failed. Did nothing happen? Did you get an error message? Did it copy but then can't be found on the iPhone?
It's impossible for us to tell you what you did wrong if we don't know what you did.
After converting the video using handbrake and adding it to iTunes, I connected my iPhone and under videos checked the video. When I clicked 'sync' it started a backup of my phone and initialized the sync but nothing was ever transferrred to the phone.