There is no official 6120 plugin for iSync. Many people are looking around, including me. And here I came across the solution at Nokia Discussion forum. Obviously, more and more people using Mac nowadays, which is great. People are start making their own software.
After install the "custom-made" plugin, I got Phoenix (N70), Penguin (E50) and Panther (6120) all synchronized with my Mac. Hooray! But... life isn't as easy as we wish... I got a Synchronization Failed.
[Panther] An unexpected error occured.
Device "Panther" synchronization failed
I ain't sure where the problem is. But on second try and I disabled the sync with Calendar, I got no error now.