You are re-installing (upgrading) MessageMaster server. Setup is asking for a license file, but you don't have a new license file to give it. You have the same license file that you provided the first time you installed.
Simply provide the location of your existing license file (usually in C:\Program Files\Objective Software\license.xml). Incremental upgrades do not require a new license file.
If you have received a new license file, due to an increase in the number of clients in your environment, by all means, provide the location of the new license file. It will be copied into the program directory during installation, overwriting the existing license.xml file if present.