In my logs I've seen that a lot of people have come to my website to get information on how to use iSync with the Nokia 5500 phone. The solution is actually quite easy:

1. Go to mactomster.de and download the iSync plugin
2. Extract the zip-file: mt-iSync-Plugin Series60.zip
3. Copy the the extracted folder "PhonePlugins" to /Library/
4. Enable the bluetooth on both the nokia phone and on your mac
5. Pair the photne with the mac
6. Start iSync and voila!

If you can't find the zip.file on Tom's website, you can download it using this link: Nokia 5500 iSync Plug-in

Please note, that this possibly isn't the most recent version of the plugin - but it works perfectly with my phone so I don't bother updating :)

I found this solution using macncell.com.