HouseBuilder comes in a zip file, that expands to be a folder and an rb file. They sit next to each other in your Plugins folder:
Amazingly it does seem to work still, though it has an annoying side effect where you get a beep sound when doing many interface actions (copy, paste, right click, etc).
On the question of a phone number to call, the license manager page (used for checking out the state of your Classic license) includes the phone number next to each license that is currently under support. In the original email that gave you your SketchUp Pro license there is the number as well.
I’m not actually sure which email SketchUp Pro subscribers get the phone number in, but one of them.
The phone number, or email contact, is meant to be for people having problems downloading, installing, or activating SketchUp. It isn’t a consulting service where someone would answer ‘how to’ questions. I would love to be doing that, but one call talking someone through how to do something could keep 10 people waiting who can’t even get SketchUp going.
My record for a phone call so far is two hours, and that was just to talk someone through downloading and installing SketchUp on a Mac. As it was exactly about downloading and installing, and activating as well, I had to stick with it, but talking something through computer basics can be time consuming.
Anyway, a good thing about the forum is that a few minutes of our time can help out a lot of people, and I don’t have to dedicate my time to wait while you read and understand the solution!