If you are not an Admin of the project or if you are seeing this message, please contact your Jira admin for access.
If you are a team member / contributor -
You can access Waterfall for Jira from the Apps menu. Just click on Apps>>Waterfall for Jira.
It will take you to Projects Overview Page, where you can add all the projects that you have access to from the list.
If you are a Project Admin -
Go to a Project and Select Waterfall for Jira.
Toggle it on, it will give you access to the project with all the features and the project fields