Control Panel > Portal Settings

"Forums to pull news from:
Only displays on the Main Portal Index page. You may specify multiple forums to pull news and announcements for the main page."

To the right is a box of all forums from which to choose. Although the instructions state one "may specify multiple forums" there does not seem to be a way to do this with the simple drop down box.

If you hit Control while you're clicking, you can choose more than one.

Or you can click and drag too