Select the proper link for your version of Civil3D. This will download the executable to install the Trimble Link extension into your Civil3D software. Trimble Link allows you to interface directly with Trimble devices and filetypes within Civil3D. If the Trimble Link tab does not show up automatically after installation, you may need to run a command in Civil3D to load the menu.
You may be required to register Trimble Link before you can use it. The registration form can be found here.