The FAQ tab for the plugin has a few tips
The left hand list of form elements are the different types of fields you can use (basic text, a drop down list, a radio button selection and a calculated field)
The top right hand bit is about buttons and the bottom right hand bit is for any non interactive bits you want in your form e.g. headings, explanatory text.
So for a basic form, click on Input and drag an instance of an Input field onto the Form elements list at the far right. The Title is what appears next to the form and the ID is what you want to call the field to refer to it easily elsewhere
I've not figured out all the CSS stuff etc - I'm just using very simply for now until the developer does get around to a guide.
The bit I struggled with was calculated fields (my main reason for using this plugin as opposed to Contact Form 7) but there is a simple explanation in the FAQ and a link to more info there.
You can create input fields but hide them to store data you need to use for calculations
Here's a form I created that used input fields, a hidden field to store a number for monthly traffic and then some calculations:
My first calculated field (Additional Monthly Income) is an output field with the formula written as
The second calculated field has a formula of
Not a complete user guide but I hope it's enough to get you started :-)