Configurin NetworkManager with the KDE-Plasma-Applet

You received a CA-certificate (AAA_Certificate_Services.der), which allows your computer to check whether it is really communicating with the login-server, as well as the public (cert.der) and private (key.der) parts of your authentification. You do not need the file cert.p12, which combines both parts of the key. Furthermore the email you received contained a so-called Identity.
To begin, save the three .der files in a location of your choice.

Right-click on the network symbol in the task-bar and select Connect to other Networks (do not forget to activate your WLAN-adapter beforehand if necessary).

In the window which appears now, select the network MI and click on Connect.

A window to enter your authentification appears. Give a name to the new connection (in the example here MI) and select the tab Wireless Security. Under Identity enter the one given to you in the email (this entry is redundant, but the widget is not capable of importing this information from the certificate). Click on the symbol next to the User certificate field and select the location where you saved the cert.der file. As Ca certificate select the MathInstUniBonn file and uncheck the box next to Use System CA Certs. Under Private Key select the file key.der; the field Private Key Password is left empty:
Add Network Connection
Click OK.

Now the network connection should be established and right-clicking on the network symbol should show the MI connection as Active.

Configuring the proxies

Do not forget to go back to the previous page and configure the proxy-settings in your web-browser