Place the tf file in Program Files/Steam/Steamapps/-USERNAME-/team fortress 2.
Note: If you have the Swamp Pack from TF2Maps installed, it may ask you to overwrite some files. They are the same, so you can say yes or no.
In order to play the game, simply open Team Fortress 2, press CREATE SERVER, and select ClassDash in the list.
Alternatively, you can type MAP CLASSDASH in the console.