Reflective surfaces like mirrors are created by using the reflectivity feature.
Location of Reflectivity Feature
You can access the reflectivity feature from the general tab of the material editor dialog.
Enter the edit material command and click on the material you want to mirror.
Check the reflectivity feature.
As you increase the reflectivity amount, it will reflect more on the mirror appearance.
When you get the reflection you want, click the OK button to close the dialog.
The material you choose will be reflective like a mirror.
When reflectivity is not active
When the reflectivity amount is 50
When the reflectivity amount is 100