The dialog box immediately to the left of the material group box is the vertex group dialog.

Simply add a group, select the desired faces for the group in the 3d window and click assign.

Press the A key to deselect all and repeat the process for the next group.

Continue until all groups are created.

I find that naming the group the same in the Ma: , Ob: , and group name boxes ensures that the names do not have the Blender nomenclature appended on export.

When exporting select the polygroups option in the export dialog and the groups you created will be retained when the object is imported into Poser.

That should get you up and running if not PM me and I'll try to clarify any points that you are having problems getting to work as expected.

I'm working doubles at the moment so be patient if I don't reply immediately, it won't be because I'm ignoring you.