How do I view my sub domain stats in Google Webmaster Tools?

Instead of uploading the html file. Verifying your site in webmaster tool using analytic code is the simplest way.

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.

What you will need doing is adding the sub domain to Google Webmaster Tools by clicking on the "Add a site" link. Once that is done, you will be prompted to verify ownership of the site/subdomain. If you choose the html meta tag method, then you will need to add this meta tag to the head section of your webpages. For the html file upload verification, you will need to upload the html file to your subdomain root folder. Once that is done, you will start seeing your sub domain stats.

Bottom line is, if you don't manually add the website/subdomain to GWT, you won't see the stats.