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Simply click once to open the Managed Software Center,
scroll, if needed, to the VueScan icon, and click on the 'Install' button. You may have to wait a few moments for the installation to run. If the (If the VueScan item in the menu says 'Remove' the software is already installed, and can be found in the 'Applications' directory on the local hard drive.)
You may have to wait a few moments for the installation to run.
When the installation finishes, close the Managed Software center (by clicking the red 'close' button at the upper left), and click the icon for the 'Applications' folder in the dock,
and then click the VueScan icon there to start the program.