Long press on the home screen to pop up the Add to Home screen menu, choose Widgets, Analog Clock. That's the simplest way. If you don't like that clock, just go to the market and search for clock and you'll see tons of different clock widgets. Once you install any of them they will be accessible through the same widget menu.
It features the flip clock that comes with Sense UI (Eris and Hero) and also includes the weather like the HTC would come stock. Its a great widget and for $1.49 really is worth the money. I recommend it to everyone that sees my phone.