As of the time of this writing, Ushahidi's "Official & Mainstream news" section only allows RSS-style news feeds. This requires an existing RSS feed for the particular topic that you're monitoring.
One way to set up a custom RSS feed is to use Google News.
1) Setup a google account.
2) Go to http://news.google.com
3) Search for the type of stories you want, example "Oakland Riots"
4) Click the link at the bottom of the page that says "Add a custom section for Mehserle-verdict to Google News". This will add the custom feed to the list of feeds in the left column.
5) Click on the new feed (e.g. "Oakland Riots") in the list of feeds in the left column
6) Click on the RSS link / icon at the bottom of the page.
7) That url is your custom news feed - add it to Ushahidi in the admin/manage/feeds page
You can create different feeds for different search terms