If you alt-click (I think)  on the create metaball icon it creates a negative one! Top tip is to instantly assign your negative meta to a different selection group (changing it's wireframe display colour in the process) and then duplicate to produce further negative metas. That way you can easily keep track of which are pos and which are neg.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.