The download will start automatically, if it does not you can download it manually from here
Attention: If you get a message saying the downloaded file is damaged then you need to allow third party apps to be downloaded in Gatekeeper.
Go to System Preferences > Security & Privacy > General
From the bottom select the Anywhere
After this download uMark manually
. Once uMark has been downloaded and run you can turn the setting back to Mac App store and identified developers
How to uninstall uMark?
Go to Applications
in Finder, find uMark and drag it to Trash.