http://www.renderosity.com/mod/forumpro/showthread.php?message_id=1154675&ebot_calc_page#message_1154675

The link above is an old forum posting that contains some metaball info. In particular there is a picture that shows how metaballs react to each other.
I think it probably says it in there but to create a negative metaball you hold down the shift key when you create.

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2604276

This second link is another old forum post that has a short tut on mapping texturs to metaballs, specifically trying to get an image texture to map in a specific location or direction.

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2605592
Another in the series of discussion on mapping textures to metaballs.

Hope those help.

I'm using Bryce 6.0 as far as I can tell..not sure what patches have been applied or not applied.