If you have concurrent license, you need to connect to License Manager from ArcGIS Administrator.
Open ArcGIS Administrator > Choose the appropriate license > (If concurrent use license), Click CHANGE under License Manager > Define the License Manager ("LOCALHOST" if the License Manager is installed in the same machine).
N.B.: For Single Use Licenses, License Manager is not required.
How To: Connect to a License Manager for ArcGIS Desktop 10x via ArcGIS Administrator
If, in fact, you have a Single Use license instead of a Concurrent Use license, check out Setting the software product and license environment and Authorizing ArcGIS single use products and features.