How can I pick a default “from” email address for my Agent?

To choose a default "From" email address for an Agent, navigate to Manage --> Agents, click Edit for the user you wish to modify, scroll to the "Hooks for Email" area, click the dropdown for "Default From Address" and select one of the email addresses shown.  Scroll to the bottom and click Save.
NOTE:  The email addresses that show up in the dropdown menu are addresses that have been set up in Configuration --> User Settings --> SMTP Configuration.  If you do not see the desired email address in the dropdown in Manage --> Agents, proceed to set up the SMTP Configuration that you want to see in the dropdown.