HEAT grill to medium heat. SHAPE meat into 6 (½-inch-thick) patties. Grill 4 to 5 min. on each side or until done (160°F). MEANWHILE, microwave VELVEETA® and RO*TEL in microwaveable bowl on HIGH 5 min. or until VELVEETA® is completely melted, stirring after 3 min. PLACE burgers on bottom halves of buns; top each with 2 Tbsp. VELVEETA® sauce. Cover with lettuce, tomatoes and tops of buns.