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.