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 option.

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.