Use the hand command on the bag to show its contents, and then drag the item to the bag's image on the right where your character used to spin around. The item will move from the bag to your inventory.
In addition, to "leave" the bag, click on that image you dragged the item(s) to.
To drop an item, press the right mouse button and select the hand with palm down.
By the way, pressing F1 could help you with some of these issues.