Control Panel > RSS Feeds > Multiple Forum RSS > Edit

"Source Forums
You may choose multiple forums as your source."

There appears to be no way to select "multiple forums as your source" from the simple drop down menu. It seems only one can be selected.

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

I have no problems in 7.0 (just 7.0, no fancy minor releases) adding multi forum RSS feeds, hold down control and click in the list box.