Under the 'Processor type and features' option, make sure 'Symmetric Multi-processing support' is turned on. If you're using an existing smp config, it's probably already turned on, but it pays to go through and check *all* the options (even if you don't think they've been changed) to be sure. It's a real pain to spend 30+ minutes doing a compile, only to find that something is wrong, and it doesn't work.
It's also a good idea to leave your old kernel alone. You can name a kernel image anything, you just have to reference it in the lilo.conf. Leave the entry for the existing mitel kernel, and copy it, changing the relevant details to your new kernel (which you should copy into /boot).
If this is all greek to you, then I suggest re-reading the howto you mentioned, as well as the kernel compile howto on linuxdoc.org