"...and assigned the material (tongue.)"

Hmm, don't assign the tounge material.
Use it only to select the tounge mesh, but then assign a "New Material" to make the new group permanent.
This will give the tounge a garish color, but once you have finished your morph, export it as an object and use it with a copy of your original Hiro.

Also move the magnet zone around and/or rescale it.

There is no need to add the new group to the magnet itself, that's why Poser doesn't give you that option.
As long as it's added to the magnet zone and it is inside the magnet zone, you can deform it with the mag.

Hmm, how many "undo's" have you set in your general preferences ?
These eat up a lot of money.
Set them to 3 to 5, as the morphbrush needs all the RAM it can get to work on hirez figures like Hiro.