A few days after passing your exam, you should have received an eMail containing a code and explaining how to use it to register at the MCP web site. If you registered, log in to the MCP site and follow the link at the bottom left of the screen to download
your certificate as a PDF or XPS file. If you wish, you can also order a physical certificate to be sent to you, but this requires paying a fee. Microsoft no longer ships the certificates automatically.
If for some reason you did not receive your registration message, then you will have to phone the Regional Service Center and explain the problem. They should be able to resend the code to you.