Here is this straight forward and easy to follow documantion about ISS-PPM integration.
After all these configuration to not get 403 forbidden error please consider below configuration notes;
Please make sure that the IIS port is listening on the same port as configured in the server.conf in BASE_URL-parameter and that this port is not only open but also different to the one set for EXTERNAL_WEB_PORT configured in the same file and also in the workers.properties-file.
Also check the ISAPI:
1. Start | Setting | Control Panel | Administrator tools | IIS Manager
2. Click on the “(local computer)” | Web Services Extension
3. Check to see “All Unknown ISAPI Extensions” has “Allowed” status.
If you know more configuration trick about this topic please inform me.