Go to Setup -> Install/Activate Extensions
Here you will find a table of installed extensions and available extensions that can be installed.
Click to green cloud icon on the right to install extension.
After installation please choose Activated for 'your company name' from the list on top of page
Here you will have to active the extension by mark the coressponding checkbox and press Update
After active the extension you will have to give Access Roles permission to access the extension by go to Setup -> Access Setup.
Logout then Login again for the extension to work.