The Circuit Playground bike light is meant to be rear facing. It doesn't generate enough light to illuminate the path in front of you.
You can mount it any old way depending on what your bike seat looks like and what other items you might have. The seat and/or seat post are good ideas. But could be a rear fender, storage rack, pannier, etc. Just be careful not get any loose wires caught up in the tire spokes.
Here's what I came up with using one of those small little under the seat packs.