How do I view my sub domain stats in Google Webmaster Tools?
Bottom line is, if you don't manually add the website/subdomain to GWT, you won't see the stats.
Just repeat the same for your sub domain that you have done for your main domain.
I will suggest you to add analytic for sub domain as well so that you can track the traffic. then add sub domain in to the webmaster and verify it using analytic. I this way you can track traffic as well as the status of you sub domain.
Instead of uploading the html file. Verifying your site in webmaster tool using analytic code is the simplest way.