Start with opening iTunes and bring up "Preferences" and select the green "Store" icon;
|iTunes Store Preferences > Store Tab|
If you want to check it's working just go to a HD item in the iTunes store and take a look at the settings on the left;
|Alcatraz, Season 1 Showing 1080p HD Download as Default (click to enlarge)|
To go straight to the option screen from the TV/ Movie screen just click on the "(Downloading xxx)" indicator on the left, it's actually a link and will take you straight to the settings page to change your option.
*- Disk space permitting of course! A 22 episode series HD will take you around 3.5GB (SD 600mb, HD 720p 1.4Gb, and HD 1080p 1.6Gb) per episode. That's around 80Gb for the complete series. Looking at West Wing that's over half a terabyte for all seven seasons for 1080p!