If you want users to see a Registration Agreement on the Registration page of your mojoPortal site, you need to create a text file named:
en-US-RegisterLicense.config
and place it beneath the siteroot/Data/MessageTemplates folder.
You can also create translations of your agreement for other languages by replacing the en-US with the abbreviation for your language. If for example you create one named:
ru-RU-RegisterLicense.config
with a Russian translation of your agreement, then user's who have their browser language set to Russian will see that version instead of the en-US version
If no file is found for the labguage preference specified by the browser, it will fall back to showing the en-US version.
The content of your registration agreement can be plain text or html. To come up with verbiage for your agreement I suggest reading agreements on other sites.