If you do not see the "Add to Home Screen" option after you have opened the Mobile Gallery App installation link, you are most likely viewing from an unsupported browser (i.e. using the Gmail app on an iOS device, or the Twitter app from an Android device).
If you are using an iOS device, try copying the installation page link and opening it in the Safari browser. If you are using an Android device, copy the link and open it in the Chrome browser.
Here are some step-by-step troubleshooting guides for common scenarios. Please follow the instructions for your specific device type.
I use the Gmail app and I clicked on "Install App" from an email.
If you clicked on "Install App" from an email and you are using the Gmail app, the link will open within Gmail app browser which does not support the Add to Home Screen option.
Simply click on the Safari browser icon at the bottom right of the navigation bar, it will open the installation page directly in Safari. Once in Safari, click on the Share icon and "Add to Home Screen" .