The Description says “The Signal” and “The Writer” are included, but they're not in the Bonus Content section. How do I get those episodes?

I am downloading this now ... I assume the DLC bonus expansions are already installed and added into the main game download.

At least on the Steam version, the DLC had to be accessed through the chapter-select menu (I can't remember the exact wording, but it was the option where you could replay individual chapters). The DLC is available from the start, but contains major spoilers from the main game so it should be played after completing the main game :)

If you go to the "Episodes" section of the main menu, you will find them there. My brother, who has beaten the game for 360, recommends leaving them be until after you've completed the main game.

I recommend to play the bonus content after the main story, too. You revisit some Levels and everything what happens in the DLC is based on them. Otherwise you don't get the whole stuff behind the DLC and you get some spoilers. ;)