Apple doesn't yet support the K750i (it will) and so to get iSync to recognize it we have to add some information about it to a file inside the iSync package.
The first thing you're going to want to do is quit iSync if you have it open. Next, locate iSync.app inside your Applications folder and right-click on it; choose "Show Package Contents."
Once inside you're going to want to get to a Resources folder that is buried rather deep in the directory structure; the following is the full path (realize that it was split to fit on the page):
Once inside this directory, you're going to want to open the MetaClasses.plist file with your favorite text editor. You'll notice that the file is mostly made up of blocks of markup that look this:
<key>com.sony-ericsson.K750</key> <dict> <key>Identification</key> <dict> <key>com.apple.gmi+gmm</key> <string>Sony Ericsson+Sony Ericsson K750</string> </dict> <key>InheritsFrom</key> <array> <string>com.sony-ericsson.irmc2</string> </array> <key>Services</key> <array> <dict> <key>ServiceName</key> <string>com.apple.model</string> <key>ServiceProperties</key> <dict> <key>ModelName</key> <string>K750</string> <key>PhoneIcon</key> <string>SEK700.tiff</string> </dict> </dict> </array> </dict>
Somewhere in that file you'll want to paste the above markup (I put mine after the K700 entry). As you may or may not have noticed, I pointed the K750i icon to that used for the K700; the icon kind of looks like a K750i and until there is true support for this phone from Apple (at which time Apple will supply the icon), it's a good stand-in.
All that's left for you to do now is pair the phone with the computer. I initiated this from the computer, but the opposite way should work just as well. After the pairing is done (or during the process), iSync will open and you should be able to synchronize.