I tried with Firefox, and spacebar seemed to start the video.
Make sure you've got the latest version of your browser, and maybe try disabling any plugins/extensions if you have any. Sometimes that stuff affects video playback.
Otherwise, you just have to send in a ticket to the help center and wait for their response, if you haven't already. As always, make sure to provide as much info about what system and browser you're using and what videos are giving you problems, etc.