First off, before using MissingSync, back up your stuff and sync your devices the way you have been to make sure all your info is current. See here:
http://www.markspace.com/support/kb.php?x=&mod_id=2&id=2674
I remember having to be sure to turn off Palm's HotSync and Mac's iSync in MissingSync's conduits window since they caused conflict with MissingSync's conduits. There's a lot of info about conduits here:
http://www.markspace.com/support/kb.php?x=&mod_id=2&root=114
that may help if you are having trouble getting up and running.
Also during the intial process USE THE MANUAL/HELP INFO that came with MissingSync! I know this goes against the grain for many men, and I'm sorry to be so hard on your sex, but the initial getting started process is clearly laid out and they explain the pop-messages that ask about how you want your data handled with the first sync.
I missed this the first time, so I uninstalled MissingSync and reinstalled, so I'd get the messages and interpret them correctly (they only appear with the inital sync). The little check boxes are important and I didn't check them the first time and all my data was insanely duplicated!
Take a little time and familiarize yourself with the manual first. It will save you loads of time in the long run!
Just a tip from your Aunt Deb!