You apply a pattern fill. Read the online help.
Maybe you're looking for the Scribble-Effect.
Want to do a cross-hatch? Apply two fills with different angles of scribble.
If you are looking for the Hatch Effects (aka The Ink Pen) that was in Illustrator 10, forget about it, it was remove in CS and somehow replace with the Scribble Effect even if the results and functionality is not similar.
Also take a look at http://scriptographer.org/ for some way to fill shape with pattern elements via scripting.
Well it was actually a filter which should have remained as an effect but I think it was actually mistakenly removed because the scribble effect
was thought to do what the Ink filter did or satisfy most users.
Why it was not brought up to date once the mistake became apparent is still a mystery?