Another part of the Mass Action feature is triggering an Action Plan from Contact List.
All you have to do is select the individuals you want to reach out to then click on the "Selected" button, you will see the list of Mass Action options. Click on "Trigger Action Plan", a pop-up box with the list of your Action Plans you can choose from will appear. Once you know what Action Plan to use, click on "Trigger Now" and that should be done.
You will be directed to the page where you can monitor the progress of the Action Plan you just triggered.