The Inspector is nice, but its MOP code is a thin layer of abstraction over the MOPs of SBCL and OpenMCL. If you're not running one of those, it gives you a "No MOP" error. Matthieu Villeneuve's patch adds support for CMUCL, but it seems like suplication of the work already done with CLIM-MOP.
Here's a patch that deletes the compatibility code and changes everything to use CLIM-MOP. It works for me on SBCL 0.8.16. If it breaks anything, let me know.
-Peter