The XM module for the AXXESS interface plugs into the AXXESS module, which then plugs into the TAPE/MD slot on the bottom of the HU. The port on the back of the HU used for the OEM sirius is left empty.
With the AXXESS installed you'll get to the XM by pushing the MEDIA button. Actually some people said it sometimes works better to hit FM/AM, and then MEDIA to switch modes without any problems.
Hope that answers your question. I think there's some questions about whether the AXXESS module works on 2007 models, or more specifically Clarion HU's. I believe they're working on the problem.