Extra bag

Sorry, I know it sounds stupid....how do I open the bag I am carrying?

Go to your inventory, you should see your backpack and then any other bags, pouches can be clicked from there to see the contents.

Of course you have to "mount" the bag or belt pouch first. If it is just taking up a slot in your backpack, it is useless. Equip it first and do as Sir Stavanis suggested.

how do you equip it?

Next to your backpack icon there is room for extra spots, but they are grayed out. The closest grayed out slot, the one that is next to the backpack icon, is your beltpouch slot. Place the beltpouch there and you are set.