Once you have signed up or purchased, grab your credentials and log in, (should be in an email).
Get started by creating a new project by clicking on the 'Create New Project' button on the top right.
Either create one from scratch, or from a template, the choice is yours
Start adding content by dragging a widget from the widget tab, (you can also change the library from 'Basic Widgets 2.7' to something else), or from the widget on the top of the screen. Just drag and drop!
Everything else to format widgets, add actions, and check out your outline is on the right side of the page.
When you want to see how your project behaves in the browser as your reviewers would see it, just select 'Play'.
That should get your starting prototyping. If you want to add requirements, just select the 'Manage' tab at the top.
If you have any questions, feel free to contact support. There's also a fair amount of tutorial videos here.