Answer
The ArcIMS Installation Guide available on your ArcIMS CD, documents the steps involved in configuring ArcIMS for use with WMS. Refer to Step 3: Install ArcIMS, "Installing ArcIMS custom Application Server Connectors".
Additionally, the ArcIMS Help system provides details about how WMS requests are handled through the WMS connector. This system is accessed by selecting the Help menu and clicking Help Contents from any of the ArcIMS applications--Administrator, Author, Designer, or Manager.