Just run the .deb file for it (should be in the same place as the other files for it), presumably after you've run all of the required .deb files. I don't use multiboot, so I can't say about what you need to do in the .item file porselinheart mentioned. It should be mentioned countless times if you (power)search "power kernel multiboot" (Optionally include "not showing up" - since this is a problem people have had with power kernel since multiboot came out.)