Maverics VPN to Windows 2008 SSTP with Certificate (PPTP)
Apple's support article, OS X: Set up a connection to a virtual private network, walks through the steps:


  
  Choose Apple menu > System Preferences, then click Network.
  Click Add (+) at the bottom of the network connection services list, then choose VPN from the Interface pop-up menu.
  Choose what kind of VPN connection you want to set up from the VPN Type pop-up menu, depending on the network you are connecting to, and
  give the VPN service a name.
  Enter the server address and the account name for the VPN connection.
  Click Authentication Settings, then enter the user authentication information you were given by the network administrator.
  After entering the user authentication information, click OK, then click Connect.
  


Below is a screen shot showing the PPTP VPN settings. Authentication Settings… contains the option for applying your certificate.