Hey Dave,
Thanks for contacting our customer support!
Here are more exact instructions for using our inbox feature:
1.) Create an account with Email on Acid.
2.) Verify that the body contains your HTML email.
3.) Send your email to “username”@emailonacid.com (insert your username and remove the quotes).
4.) Log into your Email on Acid account - you will find your inbox in the top right corner of the member Dashboard.
5.) From the dropdown, select the “Run a Test” option and you will see the html of your email pre-populated in the content section.
6.) Make your SPAM and Email Client selections and run the test.
It sounds like you made it all the way to step 5, did you see the HTML content of your email pre-populated for you? If not, please verify that there’s HTML content within your email by sending yourself a quick test.
Let me know if this helps 
Miki