Click on the link below to download the isync plugin for the Sony Ericsson K850i. Save it to your desktop.
Once the zip file has finished downloading, locate it on your desktop and double click it.
The zip file will then extract a folder onto your desktop called SonyEricssonK850. Double click this folder to open it.
Within this folder is another folder called SonyEricssonK850. Double click this folder to open it.
You will then see two files: instructions and SonyEricsson-K850.phoneplugin. Click to select SonyEricsson-K850.phoneplugin then press Command+c on the keyboard to copy the file.
Select Go to Folder... from the Finders Go Menu.
Type /Applications/iSync/Contents/Plugins/ApplePhoneConduit.syncdevice/Contents/PlugIns/ into the textfield and then click the Go button.
Now press Command+v on the keyboard to paste the Sony Ericsson K850i plugin into the right directory.
Now when you attempt to add your K850i to iSync it will actually work!