When you connect your Nokia 770 to the PC with the provided USB Connection the Tablet should indicate that it's switched to USB Mode and therefore you should be able to find the devices MMC Card as a removable drive option on the PC. (I think you already managed to do this?)

As long as there is free space on the drive you wish to add data then cutting and pasting music from the PC to the Nokia 770 should just work.

My recollection of Nokia 770 was that we could only transfer data to the external card of the device in USB Mode?

So when you say it did not work, could you be more specific i.e. What operating system, what you did to where, what were the messages when it did not work?