Here's a quick and easy procedure you can use for Windows 7, as long as the app in question uses the global Windows certificate repository.

• Fire up MITMProxy, and configure Firefox on the Windows box to use it. Browse to the target domain, and you should see a big warning about an untrusted certificate. Use Firefox to export the certificate ("Add Exception", "Get Certificate", then "View", tab to "Details" and click "Export").
• From the command console, fire up certmgr. Select "Trusted Root Certification Authorities", then on the top menu, "Action", "All Tasks", and "Import". When prompted, select the certificate file you've just saved from Firefox.
• And that's it - your certificate should now be trusted for that domain. Happy pentesting.