It appears that Mac OS X’s iSync does not natively support the Nokia 6300. A closer look revealed that several other Nokia phones that run the exact same operating system as the Nokia 6300 are supported in OS X, so therefore, the 6300 is actually supported. We are going to modify iSync, so its also seeing the 6300 as a supported phone.

In order to get this device working with iSync, follow these steps (it doesn't seem to work with Snow Leopard 10.6 !):

Quit iSync if its running. Right-click on the iSync application in your /Applications folder, and select “Show Package Contents”. Navigate through the folders to Contents -> PlugIns -> ApplePhoneConduit.syncdevice -> Contents -> PlugIns -> PhoneModelsSync.phoneplugin -> Contents -> Resources. Back-up the MetaClasses.plist file by clicking on it, and select File->Duplicate from the Finder menu. Next, open MetaClasses.plist with a text editor.

The Nokia 6280 runs Series 40, 3rd edition with Bluetooth and USB, exactly the same as the 6300. We're going to copy the XML entry of the 6280 model, and changing it a little bit, so the 6300 is getting recognized by iSync.
The 6280 XML entry starts with “family.com.nokia.series40.3rdEd.bus.usb-bt”. So make a copy of it’s XML entry, and replace 6280 to 6300, and insert the new entry below it. Here is my Nokia 6300 definition or just use my complete MetaClasses.plist file.

In order to get an 6300 icon displayed in iSync, we need to add a tiff file to the same directory as the MetaClasses.plist
The icons are 60×60 pixel tiff files with transparent back-grounds. Here is the icon file that I used for the 6300, just put it in the same folder.

Save and close the MetaClasses.plist file and start up or restart iSync. Make sure that Bluetooth is enabled on your phone. Select “Add Device” from the iSync Devices menu. iSync should find your Nokia 6300, and display the icon that you've added.

For fixes regarding Birthdays and Contact Photos, take a look at this blogpost at dabgotra.com.