Where do you add small guard exp?

The Guard building, (between Drill Grounds and Bank), which is unlocked upon reaching castle level 10. This facility allows you to recruit and assign "Guards" to one of any of the 8 available positions. When you've successfully assigned a Guard to a position, you will get its boosts. The boosts can be increased by leveling up your Guards, -- which is where you might want to use this "small guard exp" item that you were asking about.