By default, when you install the Zeplin for Jira app, it will be enabled for all your Jira projects.
To enable the app for certain Jira projects, you can select “Zeplin for Jira Settings” from the Apps menu up top. From there, you can click on “Select projects” and select which projects you want to enable Zeplin for.
☝️ If you don't see the "Select projects" option under Zepllin for Jira Settings, make sure that you have Jira admin privileges. Only admins of your Jira instance can update this setting.