Run the FSDT Live Update, as discussed so many times in this thread and other threads, with all users confirming it fixed the problem.
If it hasn't for you, it's possible (as discussed in this and other threads too), it's possible that in your case, the antivirus or the firewall is preventing the Live Update program from downloading the files, possibly because it mistakenly assuming it was downloading something dangerous (it obviously doesn't) so, the solution it's always the same: disable every antivirus and firewall and run the Live Update again. You might also need to exclude the whole Addon Manager folder from scanning.
If still doesn't work, try this:
- Uninstall GSX
- Reinstall it using the current version of the full installer.
