Hit this sequence info/telephone/phonebook. At this point select the name you want to add the voice tag to. From here click the 'edit' button. From there on the left side of the screen you will see a 'double arrow' pointing down. Hit that and you will see something like 'voice dialing' or something like that. I think you can click 'edit' here. This will give you the option of adding/editing/deleting a voice tag.

I'm recalling this from memory so the wording/buttons may be a little different.

Just a bit of advice. I find it much easier to program the speed dial buttons and use those while driving.