The code he wrote has an error. The if statements should read:

if (theCollider.gameObject.tag == "Player")

He never defined other so Unity has no idea what to do with it. Try this simple fix and you should be good.

one way is you could have two scripts such as the one for the gui which you attach to an empty gameObject. I'm assuming you know how to make a gui box already. Name the object e.g. Info

Then make a empty gameObject at the building and attach a collider and make it a trigger. Then write something like:

var info : GameObject;

function Start () {
info.active = false;
}

function OnTriggerEnter(theCollider : Collider){
if(other.gameObject.tag == "Player"){
info.active = true;
}}

function OnTriggerExit(theCollider : Collider){
if(other.gameObject.tag == "Player"){
info.active = false
}}

haven't tested it yet, if there are any problems please ask.