In ArcGIS Desktop, extensions need to be enabled to use them. If you don't have the extension enabled that your project requires you will receive an error telling you that the tool is not licensed.
This error message is a little misleading as in most cases Bren has licenses available for all needed extensions. By enabling the extension manually ArcGIS will check out the license it needed from our license servers and allow the tool to run.
ESRI maintains current documentation guiding you on how to enable extensions, you can find these instructions here:
In general you can enable the extension needed using the following procedure:
To enable an extension, select Extensions from the Customize menu in ArcMap, ArcCatalog, ArcGlobe, or ArcScene (the application that comes with the ArcGIS 3D Analyst extension).
The Extensions dialog box lists the extensions currently installed on your system that work with the application you are using. For example, if you are using ArcCatalog, you'll only see extensions listed that work with ArcCatalog. Extensions are listed in this dialog box whether or not you have registered them or whether or not licenses are currently available for them on your License Manager.
To enable an extension, check the box next to it:
If the extension is successfully enabled, the box will appear checked.