Once you put the pack.gbx file where Arkive said, restart the game, and you can click on an empty station in-game and choose to install "from disk". And then find the TrackMania 2 titlepack in the group of (what appears to be) DVD jewel cases that appears.
Normally you would be able to install from the Store in-game, but Nadeo hasn't updated the TM2 titlepack and released it in the store yet. So we can only use the old version. That's why it's linked here. Multiplayer doesn't work in it currently, but you can still use it to build maps.
Regarding the Stadium64 base.... you can drop that in
Documents\ManiaPlanet\Maps\My Maps\
And then in-game when you open Stadium, it'll show up as a map you can edit. Then just save it with a different name. You can always use it as a base/template map.