Which version of Photos do you have installed? You need Photos 3.0 or 4.0 to be able to use project extensions.
Not all project extensions can convert existing Apple Photos Projects. The Mimeo extension seems to be currently the only one. You will the extensions, that can convert an existing project, when you open the project and click the "Convert" button in the upper right corner of the toolbar. This is what you should be seeing inPhotos 3.0 on High Sierra or Photos 4.0 on Mojave.

It may depend on the country. Mpix is only available in the United States.
https://support.apple.com/en-us/HT208920
In Germany I can see only Mimeo there, when I try to convert an existing Calendar. And the Mimeo conversion is removing all custom date entries I added previously in Photos.
You could save your calendar as a PDF and upload the PDF to Mimeo to get the Calendar exactly as you prepared it. Just click the "PDF" button in the toolbar with your calendar project selected in the sidebar.
See this document on how to save and upload the PDF to Mimeo Ordering Books and Calendars on a Mac that cannot be upgraded past El Capitan