Tap a Mixel to select them, then tap anywhere on the level to run there. Tap on any enemies you'd like to attack. Every Mixel has unique powers you can use by tapping the button next to that Mixel's picture at the bottom of the screen. Once you start building defensive towers, you can tap on any empty spot to build a new tower, or choose a tower to upgrade or move. The Mixing Lab lets you mix together different items to figure out powerful new combinations.