Step 1: Install certificates
Double click the certificate file (or multiple files) you received by email in Outlook or Windows Explorer. Just keep the default options of each step and click next until the certificate is installed. At some point you need to enter the certificate password, you received by SMS.
Step 2: Set up certificate in Outlook
File > Options > Trust Center > Trust Center Settings > Email Security
or in German:
Datei > Optionen > Vertrauenscenter > Vertrauenscenter Einstellungen > Email Sicherheit
Click Settings under the first section, called Encrypted Email. Here you should see the email-address of the certificate you just imported. The default settings should be fine. Just click OK and close all the dialogs.
Step 3: That should be all. You can start sending signed emails right away. Just select the option to sign (or encrypt) the message when composing an email. The two buttons should be available under Message (Outlook 2007) or Options (Outlook 2010+)
Step 4 (optional): Install contact’s certificate
For encrypted email you first need your contact’s public certificate. Outlook will usually save those automatically, if you ever received a signed email from the person.
Alternatively you can always add the certificate to your contact’s addressbook entry. Right-click the name in the From box, and then click Add to Contacts on the shortcut menu. If you already have a contact entry for this person, select Update new information from this contact to the existing one.
If you never received a signed email, you can also import a public certificate from a file.
More information: http://office.microsoft.com/en-001/outlook-help/add-or-import-a-certificate-into-contacts-HA010099276.aspx