If you already had a ContactUs.com account, but its the first time you install the Contact form 7 Integrations plugin, your account might not have a default post form. You need a Default Post Form to use the Contact Form 7 Integrations plugin. If its a new ContactUs.com account, created from the plugin, a post type form will be automatically generated. The plugin, during setup, will tell you, that you need to create a Post type form. This is super easy:
Step 1 - Go you your ContactUs.com admin panel, go to Forms > press the "Create New Form" button on the top left part of the ContactUs.com admin panel > choose "Add Post".
Step 2 - Save and Publish.
You will also see options to edit your Delivery Options and Follow-Up Email Options. You can find instructions for that here.