It seems that you have requested for evaluation license for Confluence self installed version, while you are looking for Confluence OnDemand Team Calendars.
To obtain the correct license for Team Calendar for Confluence OnDemand, you need to go under your account in https://my.atlassian.com, and navigate under your products to Atlassian OnDemand > Click on Configure and from the options chose to add Team calendars for Confluence, you can obtain an evaluation license from there and it will be automatically added to your OnDemand instance.
Let me know if you face any issues.
I just did what you indicated above and mine OnDemand instance still says it is in read-only mode. I did go through the my.atlassian.com OnDemand Licensing and it says:
Thanks for signing up for Jira Ops! I’m Matt Ryall, leader for the Jira Ops product team at Atlassian. Since this is a brand new product, we’ll be delivering improvements quickly and sharing updates...
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