Sorry I wasn't specific. Here's what I mean.
To handle cargo items (to grapple, to drop, to pack, or to unpack), you must switch HUD modes (press ALT-M) and make sure you are in the cargo HUD mode. See below. Pressing ALT-M alternates between various HUD modes. For instance, to enter a vehicle, you must be in the vehicle information HUD mode. You cannot handle cargo in the vehicle HUD mode, or enter a vehicle when you are in the cargo HUD mode.
As to your specific question about grappling and packing/unpacking:
When you're in this HUD mode (I called it the cargo HUD mode for lack of a better name), come close to an item, like the flag, and press ALT-G to grapple it. To drop a grappled item, press ALT-R.
To pack an item, be sure you are in the cargo HUD mode (see above), and come close to the item you want to pack (do not grapple), and press ALT-P. The item will turn into a cargo box. To unpack, press ALT-U.
Hope this helps.
EDIT: Also, note that the commands do not work when you are in the HUD mode which describes shortcut keys for specific commands.