I've set my use key to a few different buttons, but so far I can't get it to work. Any idea why this might be?

You are using it when you are next to something that says to press your use key aren't you? It doesn't do anything unless you are next to something that can be used.

I'm trying to use it to read the frequent message pop-ups. I assume that can be done anywhere?

Well, I've not got First Encounter installed any more but I do still have Second Encounter installed and I think they use the same control setup, though I have modified my controls but I don't remember the defaults.

For me, E is my use key and it only works when I'm next to an object that I have to use (you get a message on screen telling you to use it) or when I am holding the sniper rifle as it doubles as the zoom key. For the messages you get via NETRICSA I have to press my "Invoke NETRICSA" button which I have set as right mouse button. Even though I have the option enabled to double-click use to invoke NETRICSA I still have to use my defined button to do it, double-tapping E doesn't work.

Thanks I will try this.