I'll try to help.
If I understand your question correctly, you upgraded your Mac Computer to Snow Leopard and you are now trying to download your contacts to your Mac computer, correct?
First this should have happened automatically. However, if iTunes is not set up correctly this may not happen.
What I would do is the following.
1. Connect your iPhone to your computer.
2 Open iTunes by clicking on its icon.
3. In the gray column on the left of the iTunes screen about half way down the list you should see DEVICES and under it should be your iPhone. Click on it.
4. This will open the iPhone set up page on the right screen. You will see names across the tool bar at the top just under the Apple Logo. They are Summary, Info, Apps, Ringtones, all the way across to Nike + iPod.
5. Click on Info which will open a group of menus. The first is Sync Address Book Contacts. In this menu ensure that the first button "All Contacts" is turned on. A black dot in it shows that it is on.
6. Now click on the SYNC button in the lower right corner and it should sync your contacts.
I should WARN you. If you get a question as to whether or not you want to Merge your contacts, Choose Merge! If not you may wind up with two or more records of every contact.
I hope this works for you.
