Picking Up

Move your character on top of an item or container and select "Get" from the "Action" menu. The item or contents of the container will be shown in the "Get" window. You then click on the up arrow next to each item name to pick up that item. If there are multiple items present, like 10 heal potions, then the default will be to pick up all of them. Option clicking on the item's up arrow will allow you to enter the quantity that you want to pick up.

You can also pick up all of the items you are standing on by choosing "Get All" from the "Action" menu (Command-A).