Change Individual button States in a Grouped menu

That's a basic trigger to change the state of an object. You'll only see the option to change the state to "Mouseover" if the object has a state called "Mouseover." If you don't already have a custom state with that name, you'll have to create one.

BTW, anything in the Trigger Wizard that has a dashed underline is editable. So if you're not seeing what you want/expect for part of a trigger, try clicking the options to see what's available.