1. Extract the module archive which you downloaded from the client area.
2. Upload the module to your WHMCS installation.
3. Go to the WHMCS Admin backend, click on Setup → Addon Modules and activate the module.
4. Click on Configure and do configure the values which you want to edit or leave it at the default values. At the point "Full Access IDs" you can specificate which Admins should have access to all generated PINs.
   Accounts which are not listed will have to use the search mask to lookup the pin. Admins must be specificated by their ID.
5. At "Access Control" select "Full Administrator".

If you want to use the dedicated module page, which is available under https://yourdomain.com/index.php?m=supportpin, you need to add an menu entry.
An example hook to add the menu entry is included in the module package.

On WHMCS v6 or above a sidebar widget is available, therefore you do not need to add a link to the module page. 
At the module configuration you choose via a dropdown between 2 different sidebar themes: