Many of us who upgraded their XCode to 4.3 must be facing an issue with FileMerge application which is part of svnX. After the XCode 4,3 upgrade, FileMerge application location has been changed. This is why it fails to launch from svnX.
You can fix this running the following command on terminal.
Also, you can launch the FileMerge directly from Xcode.
Xcode -> Open Developer Tool -> FileMerge.
The application binary can be found here: