viktor: You need to do it as root user. (A good rule of thumb is, if it's not inside /home/user/ you probably need root access to edit it.)

The best thing to do is to install rootsh from the app manager. This will let you simply type "root" or "sudo gainroot", and you will be elevated to root user in that X-Terminal window. If you don't have rootsh, and you locked "app manager", then that's slightly more difficult. I'm not currently aware how you'd launch app manager from the command line...

(Just tried it. Just type "hildon-application-manager" in X-Terminal, and it should launch the app manager. This way you can install rootsh if you don't already have it, and from there lets you edit the files to unhide your apps again.)