These vouchers are provided by Miles & More and users can find them in their inbox. To use the voucher you need to print it and show it in your preferred Lufthansa Business Lounge. You find the eligible ones on the Lufthansa website by selecting "Business Class" on travel class.
You can use the Business Lounge voucher with any flight operated by a Star Alliance airline.
The Business Lounge vouchers are only valid for three months from the day they are send out.