1. Create a selection of the contacts you want to add the note to by 'Searching' for them
In this case I used Search > Personal to locate all of my constituents in Texas because I sent them a mailing.
2. Once the correct contacts are selected, from the 'Data Tools' drop-down select 'Tag Selected'
3. It's just like adding an activity note to an individual record but this will add an individual activity note to all selected records.
Update: Activity notes can now be deleted in batches as well.
Click here for more information on deleting multiple activity notes.