Long pres on a blank area of your home screen, then in the menu that pops up, choose Android Widget, then if you see "clock" or similar listed, pick that.
If you don't find anything in that menu, go to the Market app and search for "clock widget", there are a bunch of free ones. Install one or more of them, then repeat the above.
Some widgets need more space on the screen than others and might not let you add them unless there's enough space in the area you selected.