In As You Like It, why does Adam say to Orlando, "Dear master, I can go no further: O I die for food. Here lie I down, and measure out my grave."

Remember what has happened up to this point. 
Specifically, remember that Orlando has run away from the court to avoid the plans that
his brother Oliver has made to kill him.  He has taken his servant Adam (who is quite
old) with him.  At the beginning of this scene, we are seeing them in the forest for the
first time.

What Adam is saying here is that he is too
hungry to walk any farther.  He says he is going to die.  That is the meaning of "I die
for food."  This phrase in modern English would be something like "I'm going to die of
hunger."  He says that he will measure out his grave because he is going to just lie
down and die.  When he is lying there, it will be clear how long his grave needs to
be.

So Adam is just saying that he's too tired to go on and
that he's going to lie down and die on the spot.