There are three ways to add parts. The first way is to select the part from the drop down menu and then select "Add Part Selected".

The second way is to drag and drop parts from the parts search screen. You can drag multiple parts by holding down the "Ctrl" key while you select multiple items. Once you have selected the parts to add, click and hold on one of the line items and drag them into the parts screen and release.

The third way, if you remember the part number, is to type the number in the "Select a Part" field and click "Add Part Selected".