Brown ground beef in large skillet over medium-high heat until thoroughly cooked, stirring frequently. Drain.
Add taco seasoning mix, water and macaroni; mix well. Bring to a boil; stir. Reduce heat to medium-low; cover and cook 8 to 10 minutes or until macaroni is of desired doneness, stirring occasionally.
Sprinkle with cheese and onions. Top with sour cream. Sprinkle with tomato.