"It is quite simple, when you add a new page, you should see a "page template" option in that dialouge box as well. You can select from all of the templates that "wetpaint" has added, or you can choose your own.

If you already have a page, but no template, go to your "settings" area of the site. Select "templates" and then select the template you want. Select all of the template's content, go back to the page, and then paste it (Ctrl+V)."