At the top, you can select an existing enzyme list or you can use the full list of enzymes (default). Select an enzyme, and you will see its recognition sequence in the text field below the list (AAGCTT). If you wish to insert additional residues such as tags, this can be typed into the text fields adjacent to the recognition sequence.
Click OK will insert the restriction site and the tag(s) before or after the selection. If the enzyme selected was not already present in the list in the Side Panel, it will now be added and selected.