So you've got to a website like http://www.someushahidi.com/ and you can't see anywhere to log in.
First, try going to http://www.someushahidi.com/login
If you see a login screen asking for your username and password, either register or log in.
If you don't, first try going to http://www.someushahidi.com/logout
If that doesn't work, you might have to delete the cookies on your browser, because Ushahidi thinks you're already logged into this site.