Saved parts can be added to the project either with the function Add Part or with the File Preview Browser. You can also add parts by drag & drop, if you pull them from your windows folder into your Netfabb window.
A selected part in the project can also be removed and deleted from the current project. This function can be accessed via the Part menu, in the context menu after right-clicking on the part to be removed, by double-clicking on the red X-icon next to the part in the project tree or by pressing the Delete key on the keyboard after selecting the part.