Because Asana forms are not available in its programming interface, Flowsana has to use another approach to identify what forms exist for a given project. To find your forms, it looks at the tasks in the project and identifies those which were generated from a form. (It examines the most recent tasks, up to 100.)
This means that you must have at least one task that has been created from that form, in order for Flowsana to know that form exists.
So if you're not seeing a form, it most likely means no task exists that came form that form. Please create such a task, and then the form name should display when selecting a form for this workflow.
Also please note that in order to use Flowsana's Forms Support workflow, you'll need to keep the "Copy all responses to task description" Asana form option enabled.