Answer:
Used left or right, I get to travel over cobblestone or gravel. Used up, I vie for sweet success, used down, I cause men great duress.
You can see nothing else, When you look in my face, I will look you in the eye, And I will never lie.
You use a knife to slice my head. And weep beside me when I am dead.
What do you have when you're sitting down that you don't have when you're standing up?
I can be used to type and point you see. But don't forget, you can count on me. What am I?
This food is a tortilla filled with cheese and sometimes chicken before being heated or cooked.