Here is a quick help explaining hotseats functions.
Number 1: Here is the game script and the hotseat is selected by default. So don't change it.
Number 2: By clicking on script parameters, you can edit some parameters as the track time, warmu-up, etc.
Number 3: Here you can select maps that have been downloaded on your maniaplanet folder (C:\Users\YourUserName\Documents\ManiaPlanet\Maps) or tracks that you created from the editor
Number 4: By clicking on the load button, you can choose maps of the Canyon campaign. It will load all maps of a level (White, Green or Blue)
So to start a hotseat, steps of the number 1 and 2 are optional (you can leave the default parameters).
The main step is to choose your tracks ( by using your tracks or campaign tracks), then click on the button "next".
Then please choose the number of players and their name and click on "start game".
I hope this will help you out.