Hi,
first of all the command $this->bbcode_second_pass_code('', '(sudo) php runme.php [mmc|nand]') is supposed to be executed on the host machine where you extracted the sheevaplug-installer on.
In order for the installer to work you should interrupt the plug's boot process by connecting the mini-USB cable and then powering it on. By quickly connecting to the serial console and pressing the any button you can drop to the $this->bbcode_second_pass_code('', 'Marvell>>') prompt.
Now you have to run aforementioned runme.php and just wait for a good 5+ minutes. You might run into
additional problems during installation.
Hope this helps. If you have trouble installing please ask in the
appropriate section and I'll try to assist further.
Cheers