Since you are getting a "boot>" prompt, then you are communicating with the second stage boot loader. Your problems, whatever they may be, are not related to multibooting, but to something else.
Since you attempt to load a kernel, and it fails to load and run, then your problem is possibly related to hardware or hardware configuration.
One of the things you may find helps is to turn on "verbose" mode in the kernel -- this produces more output during hardware discovery (your dmesg output).
Boot to the User Kernel Configurator -- use "-c" as an option on the "boot>" prompt.
At the UKC> prompt, type:
UKC> verbose
UKC> quit
This might help.