Yep, need to put in your stream key, assuming you haven't, or have put in the wrong value. Here's how to get it if you're using Twitch.tv:
-Log into the site.
-Go to
http://www.twitch.tv/broadcast/dashboards
-Click the 'Streaming Apps' link in the top-right
-Click the 'Show Key' button at the top-right
-Copy/paste the whole long string of gobbledygook that it spits out (just to the left of the button which has now changed to 'Reset Key') into the Stream Key box in OBS, under the Settings->Broadcast Settings menu. Do NOT click on 'Reset Key'.
-Hit OK until you're back to the main OBS screen again, and try to Start Stream again.
If this isn't the case (and you have the stream key entered correctly... do it again just to make sure!) please post a log of your streaming attempt so we can see the back-end errors. They're located in %appdata%/OBS/logs/ and are time/datestamped accordingly.
