Logic from the Left Coast

How can I choose to create a structured or unstructured project?

I can't see any way of selecting one over the other.

On the on-line manual at http://www.leftcoastlogic.com/blog/smartapps/smartcal-4/smartcal-gtd-handy-guide the only mention of
Unstructured Projects says

You can use Projects to simply house a list of Tasks that you need to do. For example, I have one Project called “Work” and another called “Personal.” I throw all my junk into either of these bins.

..but it doesn't seem to explain HOW to set them up.

It's very simple.  For a structured project, just be sure to complete the tasks in the order of the list. For an unstructured project, you can pick any task at any time.