Please be advised that this module in Admin Tools for JIRA version 1.5.0 (and later ones) has been removed:
As a workaround please try with Announcement Banner - to use it please follow the below steps whilst being logged as JIRA administrator:
Click on the visible only to such user cog icon and next on the "System":
Down below, on the left hand side please click on "Announcement banner" link:
Edit as desired and click on "set banner" button below it:
