Add Items to an Activity in the Item List section of the Author Site Activity Editor.
Select the green Add button to add Items - this will open up your Item bank in a modal. You can search for the Items you want by parameters like the Item Reference, Item Status, Question Type, and Tags.
Select the checkbox beside each Item you want to add to the Activity.
When you are finished, click Select in the top right corner of the modal.
Figure 1: Item selector in the Activity Editor.
Items will appear by default in an assessment in the order you have checked the boxes when you are adding them to the Activity. To change the order that the Items appear in an assessment, click and drag the Item References around in the Activity Editor. You can add or remove Items from an Activity at any time.
Figure 2: Items in Activity Editor.