When you launch ManiaPlanet, you get a screen where you've got "ManiaHome" on the left. To the right, you should see Shootmania: Storm as a slightly big "button/box".
If you click and drag left/right, you should see more "buttons/boxes" where it says "Click here to install new title" or something.
To play Elite, Joust, Heroes or any of the official modes, you have to install these titles using those.
I think this is a really unintuitive system (along with customisation of crosshair having to be done while in a server, and graphics options before you launch ManiaPlanet), but that's how it is atm.