Absences can be also added in the Time & Expenses view. Absences are kind of calendar entries so those can be also viewed in Calendar (Schedule).
You can add Absences by clicking the arrow next to Add work hours button. By doing this a menu opens and you can choose Absence as the entry type. Give the Absence a descriptive name and select an appropriate type for it. Depending on the entry you can either choose it to be All day (i.e. Holiday) or enter a Start and End time (i.e. if you get sick during the day).
If you need to modify the entry afterwards just hover you mouse over it and select what you want to do (Edit/Remove).