Activate directly in Blender 2.8
BlenderKit add-on is a part of Blender 2.8 so you can enable it in a second.
Go to preferences, then open Add-ons.
Put BlenderKit in the search box and check Add Mesh: BlenderKit asset library.
Create an account and activate by pressing Login directly in BlenderKit settings or Profile tab in BlenderKit panel.
How to install add-on in Blender 2.79
Create an account and activate it in Blender settings.
Your API key will appear on the bottom of this page after registration. Click on the API key button, copy it into BlenderKit Add-on settings and hit save.