Follow the below steps in order to get Mobile for JSM Portal installed in your JIRA instance:

 1- Download and install App from Atlassian Marketplace

1. For Server: 
2. For cloud :  

NB: For Jira 8.x and above for server instances when you click on the gear icon the "add-on" button is replaced with "Manage app".

 2- Install the app on your Mobile device from Playstore and/or Appstore

Check below links to download JSMobile - Service Management Portal App :

• Play Store.
• App Store.

 3- Login to your JSM from Mobile

Check this Link to login successfully to JSMobile - Service Management Portal