Go to the Global permissions page (type gg then Global permissions)
Check whether the atlassian-addons-admin group belongs to the Jira Administrators permission.
Click on the View Users link below the atlassian-addons-admin group and verify that the Jira Misc Workflow Extensions user (or addon_com.innovalog.jmwe.jira-misc-workflow-extensions) user belongs to that group.
If either of these checks fail, your Jira instance is not properly configured. You need to uninstall and then reinstall the JMWE add-on and check again. If the checks still fail, you need to contact Atlassian Support and ask them to fix these issues.
The atlassian-addons-admin group is only visible on the Global permissions page, and not on the Groups and Users pages.