You have to install multiboot-kernel-maemo_0.3-1_armel from:

http://repository.maemo.org/extras-d....3-1_armel.deb

It will create the maemo kernel - missing vmlinuz-2.6.28-omap1 file.

That was made for PR1.2.
It seams it still works with PR1.3 but it is not the PR1.3 kernel.

If you have PR1.3 installed you may extract the PR1.3 kernel from the bin file (credits to OptX).

Download RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin
from
http://tablets-dev.nokia.com/nokia_N900.php

Download and install the latest version of the Flasher for your PC
http://tablets-dev.nokia.com/maemo-d...-downloads.php

Execute in your PC:

Rename the resulting zImage file to vmlinuz-2.6.28-omap1 and copy it to /boot/multiboot/ directory in your N900.

Good luck