I would like to be able to change key assignments manually. The reason is that I have a Remote Wonder, and the play, ff, rw, stop and pause keys on it seem to correctly correspond to the (MM) keys listed in the key assignment window-- which is nice, I can control foobar remotely as long as it has the focus...a but I can't change the mappings. The remote keys will contol foobar, but won't populate the key field when I try to change the mapping.

I would like to change the Next (MM) key to do next or random instead of next.
And change the Previous (MM) key to do previous in history instead of previous.

How can I change this mapping?

If you do a search you might be able to find a few threads about this. Basically, even though your multimedia keys don't always show up in the key field, you can still assign a command and click "Add" as if they did. Whether this actually works depends on your keyboard, but it's work a try all the same.

You may need to uncheck "Core->Enable default multimedia keyboard support" in the preferences as well.