How do I get into the Mac Clam Mode?

Using an external keyboard and mouse put the MacBook to sleep and hook up or turn on the external display. Bring the MacBook out of sleep by tapping the keyboard and the external display should turn on.

Using Adapter & Putting Mac to sleep to display only connected Display:
If you're in mirror mode that will be the case, just because your laptop is 16:10 in terms of screen/resolution ratio whereas the TV is 16:9 (and you're outputting whatever resolution your MacBook/MBP is 1280x800 or 1440x900). If you switch to extended desktop mode you should have a fullscreen fill, but your dock/menubar will be on your main screen still.

If you use clamshell mode, you should be able to get everything on the TV and it'll scale properly.To get it into claim shell mode, using an external keyboard and mouse put the MacBook to sleep and hook up or turn on the external display. Bring the MacBook out of sleep by tapping the keyboard and the external display should turn on.