One thing I've noticed about the Maxx (not sure if it happens with other devices) is that when you download and install something with widgets, the widgets take a little while to show up in your widgets list. With One More Clock, after you install it, you have to find it in your Widgets list (go to All Apps and tap Widgets at the top of the screen), then long press the size of the widget you want, and drag it to a homescreen. I think it will then ask you to choose a skin.
If you don't see the widget right after you download it, either wait a while (not sure how long, maybe up to 10-15 minutes) or power your phone off (i.e., turn it completely off) and turn it on again.