You will need to be able to access a MMC or SD card to load the menu onto the MK3, MK2 or 3 is always required to boot the menu (it checks for the MK version first off and will give a white screen if its not found) and until you are able to access the MMC card you will not be able to load the menu from the MK3.
There also appears to be problems in the current beta with flashing the menu to the NeoMax MK3 (64+32) and the MK3 2005 512+128.
Someone else (JSS) has found that "remounting" the MMC card a few times in the menu (in the main menu select the MMC icon, press B and then press A to mount), or with some of the earlier versions of the menu removing the MMC and then putting it back in and mounting it again would allow you to access some MMC cards that dont mount up straight away.