Open Server Manager, click Manage at the top and select Add Roles and Features. Click 4 times Next until the wizard will show Select features. Then select the Network Load Balancing.
Select Add features to install the management tools too.
Click Next, then Install and wait for the installation.
With PowerShell, you do the same by typing
install-windowsfeature -name nlb -includeManagementTools
If you want to install only the Network Load Balancing management tools then:
install-windowsFeature -name rsat-nlb