Configuring NetworkManager with nm-applet (Gnome-desktop)

Sadly there are different versions of nm-applet, which require different approaches.

You have 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 and also both parts combined in one file (cert.p12). Furthermore the email text contains a so-called Identity. You will need the CA-certificate, the identity and, depending on your nm-applet version, either both other .der files or the .p12 file.
After you have saved all four files in a location of your choice, proceed as follows to determine which method your nm-applet version necessitates:

Steps in common for both versions

Right-click on the NetworkManager icon in the task-bar and select the MI network:
WLANs
(Do not forget to activate your WLAN-adapter if necessary.) A window to enter your authentification appears. Set Wireless Security to WPA2, authentification to TLS (nicht TTLS), and enter your Identity in the field of the same name (this entry is necessary as nm-applet is not able to import this information from the certificates). Click on the symbol in the CA certificate field and select the location where you saved the file AAA_Certificate_Services.der.

Differentiating between nm-applet versions

In order to determine whether you have an old or a newer version of nm-applet, click on the symbol in the field Private key and check whether you can select the file cert.p12 from the location where you saved the files. If you can, then you are probably using with a newer version; if you can only select the .der files it is probably an older one.

Newer nm-applet versions

Under Private key select the file cert.p12 and leave the field User certificate empty. If you were not told anything different, enter WLAN as Private key password.
Authentification required by wireless network
Now the button Connect should be selectable, click on it.

Older nm-applet versions

Select the file key.der as Private key and as User certificate the file cert.der. Leave the field Private key password empty.
Click on Connect.

Configuring the proxies

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