Ideally your Members are submitting excuses prior to an event happening and you process the excuses through the excuse queue as described here. In the event a Member did not submit an excuse prior to an event, an MCR administrator can manually enter an excuse on behalf of the Member. 

To manually enter an excuse for a Member, go to Event Admin and click on the event attendance report you want to manually enter an excuse for. 

In the attendance report, click the "View Excuses" button in the upper left corner of the screen: 

In the Excuses list view for the event, there will be a drop down menu next to each Member's name. Click the drop down menu to manually select the appropriate excuse for a Member(s) and click the update button to save your changes.