To set up Content Porter to work with SSO in mixed mode you need to: 

1) In IIS find "SDL Tridion" Web site, for ImportExportService application and ImportExportService/Client folder in Authentication settings set AnonymousAuthentication to Enabled (other Authentications should be Disabled)

2) In IIS find "SDL Tridion" Web site, for Webservices application in Authentication settings set AnonymousAuthentication and WindowsAuthentication to Enabled (other Authentications should be Disabled)

3) Open Web.config under ..Tridion\webservices directory and 

    under <system.webServer><modules>... </modules></system.webServer> add <remove name="SsoAgentHttpModule" />

    under <system.serviceModel><bindings><basicHttpBinding>...<basicHttpBinding><bindings><system.serviceModel> 
    - For CoreService_basicHttpBinding, StreamDownload_basicHttpBinding and ImportExport_StreamDownload_basicHttpBinding set <transport clientCredentialType="Windows" /> 
    - For StreamUpload_basicHttpBinding and ImportExport_StreamUpload_basicHttpBinding set <security mode="None"/>

    under <wsHttpBinding></wsHttpBinding> make sure <security mode="Message" <message clientCredentialType="Windows" /> </security> exists

4) Save Web.config

5) In Tridion Configuration Manager console under General settings, ensure the "Urls without HTTP authentication" contains "CoreService|ImportExportService" and "ImportExportService/Client"

6) Restart IIS, Tridion Content Manager Service Host service for the changes to take effect. After the changes login to Content Manager from Content Porter, it is now set to mixed mode.


This steps can't be found in SDL official documentation which was a big problem when I was implementing SSO.