You need to add the handling unit to the shipment first.

Expand the tree

Select the handling unit you intend to pack in and select the lines to pack in the Unattached Shipment Lines tab

There you can click the attach part command

Then you get the dialog to specify quantities to attach:

Click ok and you get the parts attached:

You can repeat and work as you wish to split quantities between handling units.

In case you have the Handling Units already in inventory they will be connected at picking.

Hope this answers your question.

//Fredrik

Thank you very much for the quick and consise reply.