Reflection is a surface property. The surface of the mirror needs to have a reflection value. To see this, first select the mirror in the Scene Tab. Then go to the Surface Pane (WINDOW --> Panes--> Surfaces). The mirror may have multiple surfaces (back, side, front, etc.) and it depends upon how the creator made the mirror model. Find the surface of the mirror that you want to have reflection and select it on the left side of the surface tab. On the right side, scroll down until you find the 'Reflection' property. Give it a value of 100%.
Please note that you will not immediately see reflection. Reflection, like some other surface properties (bump, displacement) will not come into existence until you render the scene.
Also note that, just as in the real world, reflection requires a light source. If you have light in the scene, you will have no reflection. Additionally, if you wish a specific image to appear in the mirror, you must have the angle and the subject to be reflected correctly set up, just as in real life.