The Agile Designer Viewer application allows users to view a flow's representation and test cases without being able to edit or modify them in any way. If you have a workgroup or automation builder license, then you are entitled to this feature.
If you are using ARD 2.7+, you can use ARD Viewer automatically with the new License Activation Wizard. A Viewer License allows you to use ARD in a limited capacity as described above. You cannot edit your work with this license, but you can use the license indefinitely. If you have any other license applied and then activate the Viewer license in the activation wizard, your other licenses will be permanently lost.
In ARD versions 2.0-2.6, Viewer has not been packaged in a way for customers to access it with the normal ARD license. To use ARD Viewer, the user needs to have a specific ARD Viewer license generated. You will not be able to use ARD Viewer with a concurrent license type.