A 403 error is a bit unhelpful really. Unlike most errors, it gives little indication as to what has caused it, just says you can't go there.
There are many, many reasons your server could be doing this. Firstly, double check to make sure you are typing in the full URL (including the extension, .cgi); directory browsing may be disabled, and in most cases is what causes this type of thing.
Also - another obvious one really - check your server actually supports CGI scripting. Quite a few do not, and a security measure on the server may be trying to prevent them from running to stop any malicious attacks.
Otherwise, you may be looking at a much deeper security issue, or more likely a fault in the TTX code. My advice would be to find the TTX forum or some kind of tech support for them and see if anyone else is having the same or similar problems.