You will need to have the BlackBerry Service on your boss' sim card before he will be able to use the BlackBerry.
For the installation of BES, check the installation documents found in the docs directory in the file you downloaded for BES Express. If you have any issue, came back here and search and post if you don't find an answer or go to BlackBerry Knowledge Base
or go to http://www.blackberry-forum.de
From which carrier is the 8700g your boss have? If it is a T-Mobile Germany Prosumer device it will NOT work with Eplus.