I made it it not clear, sorry! You do not boot the kernel , but you flash the kernel, only the kernel. You need the flasher utility for your PC's OS and the firmware image. You will find the informations on flashing in the wiki article in the earlier post, then run the command to flash the kernel-only on your PC, then connect the N900 to the PC via USB.

Please read the wiki, and ask your questions here, if any.