Try this:

1. Reinstall it, select "Repair" - If your OS is Vista/Win7/Win8/Win8.1, run the installer using the "Run as administrator" option and make sure your Anti-Virus software is off/disabled (just in it's interfering with the install process).

2. If that didn't fix it, uninstall the 707 Freighter Expansion Model, remove the x702 folder in the Windows Registry (found in HKEY_LOCAL_MACHINE\SOFTWARE\CaptainSim or HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\CaptainSim), then reinstall it and activate it using the Manual Offline Activation method.