I am not familiar with IBMboot manager, but I am familiar with grub. My own
computer has a boot manager (HP) that I can access by pressing escape. I then
just tell it which drive to boot, not which OS. Yours may be different. But in
any case the boot manager will start and grub wll take over once you load the
drive that grub is on.

Grub is a boot loader that will run from any bootable drive. You need to just
choose the drive to boot from your boot manager. Grub will load once the drive
is accessed. However, your system may not boot properly if you have more than
one drive depending on your setup and which boot sector grub was written to. If
this is the case, you will find out soon enough by trying to boot.

Once we know what happens we can tell you how to change grub so that it boots
properly. Bascially you need to write to a text file as root. It is located at
/boot/grub/ on the root directory of the OS that wrote grub. The file is called
menu.lst and the command to edit it is sudo gedit /boot/grub/menu.lst from the
terminal.

You need to know a bit about what the file means. The line that you will want to
edit has hd(0,1) or similar. The hd stands for hard drive and the first number
is the ordinal number of the drive and the second is the ordinal number of the
partition. Yours likely says hd(1,x) if it hangs and you will want to change the
first number to zero. If you hasve only one drive but several partitions you
will want to change second number to zero. It sounds more complicated than it
is.

This problem comes about when you use a boot manager because you are fooling
grub by changing the boot order on the spot. Grub's order is fixed and when you
make it relative then it will confuse grub and it fails to find the kernel.