Assigning Action Items to an Issue 
Use
On the Action items tab, you can enter steps, actions and questions for an issue, and designate a person responsible. The information you enter here is independent of the data you enter on the issue as a whole.
Prerequisites
You have opened the Issue Editor. You have also
entered an issue or have been assigned responsibility for a specific issue.Procedure

Use the arrows
and
on the lower left of the screen to navigate from one action item to other action items for an issue.
Choose Save to close the Issue Editor.