Mitigation Actions are Work Items. They can be assigned to specific users and managed by each user.
Log in to the HTML5 client.
Select the Risks Center workspace, Risks Grid.
Click on the Open button available on the right-hand side of the Risk Name to open the Risk Card.
In the Mitigation section, click on the MITIGATION PLAN button to open the Mitigation Plan popup.
Select the Mitigation Actions sub-tab and click either on the Create & Connect button or on the Connect to Existing button depending on whether the Action already exists or not.
When Mitigation Actions are created, a Connection will be created between the Risk and the Action (Work Item).
When that happens, the 
button available in the Risk Card will turn blue. The Actions section will therefore be displayed in the Risk Card.
Mitigation Actions are not mandatory.