iSync works right out of the box with the K750i but make sure you have the latest version of iSync. I think support for the K750 was added not too long ago (before that you could just modify a plist file in iSync to make it work).
If this doesn't work I would suggest unpairing your phone and re-pairing it. I've had to do that before to make my phone work.
To get Address Book to work with the K750i you need to modify a plist in the Address Book package. I'll look up the instructions if you're interested.
I have also managed to get the address to sync up with my K750. Only thing missing is the pictures also.
I do have one other issue with mine and that is that sometimes when I update a contact in the address book and sync back to my phone that person is then missing on my phone. I have to reset all data to get them back. This happens no matter how many times I sync.