There are two reasons why this button may not appear for you or your end users:

1. Check the DRM Policy Setting that is assigned to your user in your Vitrium account and make sure that the Offline Access field has a value that's greater than 0. For Enterprise customers that integrate with an external system, you may need to check your code. 
   
   
2. If the DRM Policy assigned to your user appears correct and you have External Service enabled on your account, then follow these steps:
   
   
   1. In your Vitrium account, go to the Settings tab 
   2. Click on External Settings
   3. Make sure the 'Remember Me' option is enabled (checked). If it isn't, enable it and click Save Settings.