Listen Port: The port your IAS server listens on (usually 1812).
Secret Key: This is the password shared between IMAuth and your IAS server. Before IMAuth can connect, you will need to open the IAS settings control panel and add a new client. This involves creating a Shared Secret, which you should then enter into this field.
Security Level: This box determines what type of encryption IMAuth will use when communicating with the IAS server. This should match one of the types you have chosen to accept in the IAS control panel. The strongest security is provided by MS-CHAPv2 encryption. Other encryption types are provided for compatibility.