Nokia 5800 is not supported by Apple’s iSync per default, so in order to get 5800 iSync compatibility you will need to install a third party plugin.
So far the only suitable plugin I’ve found is one supplied from Nokia themselves, so it should be working quite well. Download it here: Nokia 5800 iSync.
This 5800 iSync plugin allows you to synchronice using bluetooth. It will synchronize data from your Address Book such as name, addresses, etc. as well as data from iCal including events and to-do’s.
Once you have downloaded the plugin, simply install it on your Mac like any other application and your iSync is ready to support Nokia 5800 as if it was done originally by Apple.