The Participated button can be found toward the top-right hand corner of the forum when you are logged into your forum account. When this button is clicked, a pop-up will display showing you a list of topics that you have posted in. The topic list here will be ordered from most recently updated to oldest. This list allows you to keep track of the topics that you have participated in recently to view when new responses have been made.
At any time, you can display or hide this button when you are logged into your member account depending on whether or not the feature is for you. To do this, click the Profile button located in the top menu bar of the forum, and then on your profile page click the Edit Profile button. You'll be taken to the Edit Profile & Settings page; once here, click on the Settings tab. This takes you to the Forum Preferences page where you will find the Show Participated option at the top of the list. Set this to Yes if you want the participated button to display, or set this to No if you wish to make the participated button hidden. You should see your changes take effect after clicking the Save Account Settings button located at the bottom of the page.