The viewport and tree selections are separate, though when selecting something in one it selects it in the other, if possible. The viewport shows what is selected there, so you know what you are going to manipulate. You can't manipulate attachments in the viewport that aren't visible in the viewport, even if they are selected in the tree.
If you must modify many attachments under the same slot, you can drag them to a bone, so they all get their own slot, allowing them to be visible in the viewport at the same time. Then you can select and manipulate them all at once. You can select them with box select, or via the tree (filters can help there). When done you can change their parent in the tree back to the single slot.
What warmanw was saying is to manipulate the bones that the attachments are attached to, rather than manipulating each attachment.
