A couple weeks ago, we reported that Video Ad Support Comes To Google AdWords API Sandbox. We now have an update from Jeff Posnick, a Google AdWords API representative in the Google Groups thread.
In short, due to a mistake in pushing out the new API update, they forgot to push the video ad support changes live. It looks like the video ad support feature will have to wait until the next minor release of the AdWords API.
Unfortunately, I've been told that while the configuration change to support video ad creation in the Sandbox was submitted, due to an oversight in the push process it wasn't properly applied to the Sandbox environment. Apologies for the miscommunication and the delay in getting this implemented.
I'll post again when I hear that the configuration change has been properly pushed out so that you can try again.
I will keep you posted on any enhancements made to the AdWords API.
Forum discussion at Google Groups.