I want this song to play in the Game Mode, and it dosen't appear... Is it an unlockable or something? If it is, how do you unlock it?

It is unlockable. To unlock it, you must clear Mission E-13 in Dance Master Mode (Clear it on Heavy with a grade of A or higher).

and how do you get to level e 13

Well...it takes a while because if you go straight through the mode, E-13 is technically the final mission. So you have to play through enough stages of Areas A-D to unlock Area E, then clear enough missions to unlock E-13. You don't have to do all of them. Even on the shortest path that's roughly 100-120 missions you have to clear. At least they don't get hard until the end.

so, it would take me roughly 2 hours or more?

Yes. But probably a bit longer.