How to create trial account to access Salesforce Marketing cloud
A free Marketing Cloud Developer Edition was released at Connections in September 2014 but was only available for a couple of months. If you didn't sign up during that period, then unfortunately you missed out.
I enquired about the availability of the Developer Edition at Connections last year and Salesforce explained that while the public program had been discontinued, free Developer Edition accounts are still available, by request.
I would suggest you contact your Salesforce Account Representative to enquire if you can get access to an account (I'd suggest that you also explain what you intend to use the account for).
In the above link, you can click on any of these to create trial accounts.