Here are the instructions to get iSync Version 2.4 (501.2) (on Mac OSX 10.4.9 - Build 8P2137) working with Nokia E50. The Bluetooth pairing is smooth and should not create any problems, but natively you are not able to Sync with this device.
In the Terminal:
$ cd /Applications/iSync.app/Contents/PlugIns/
$ cd ApplePhoneConduit.syncdevice/Contents/PlugIns
$ sudo cp -R Nokia-E61.phoneplugin Nokia-E50.phoneplugin
$ cd Nokia-E50.phoneplugin
Change all occurrences of E61 to E50 in Info.plist:
$ cd Contents
$ mv Info.plist ori.Info.plist
$ sed 's/E61/E50/g' ori.Info.plist >Info.plist
$ rm ori.Info.plist
Read on for the last few steps...Change all occurrence of E61 to E50 in Resources » MetaClasses.plist:
$ cd Resources
$ mv MetaClasses.plist ori.MetaClasses.plist
$ sed 's/E61/E50/g' <ori.MetaClasses.plist >MetaClasses.plist
$ rm ori.MetaClasses.plist
Change the value of com.apple.usb.vendorid-modelid in MetaClasses.plist from 0x0421/0x044D to 0x0421/0x04C9:
$ mv MetaClasses.plist ori.MetaClasses.plist
$ sed 's/0x044D/0x04C9' <ori.MetaClasses.plist >MetaClasses.plist
$ rm ori.MetaClasses.plist
You may also want to change the icon appearing in iSync to match your phone. The image is com.nokia.E50.tiff. Respect the size, format (tiff), and the compression (LWZ) of the original file.

