In the left box, either double-click on the items you want inserted into the open document, or select an item and click on the (>>) button , to insert it over on the right side. All the items in the right side of the dialog box, when you click the 'Apply' button, will be inserted into the open document.
At any time, before clicking the 'Apply' button, you can select an item on the right side, click on the (<<) button to remove it.