The selection of additional resources does not use spatial selection but instead an ordinary Access form. You can only select one single record at a time.
Use the record navigator on the bottom of the form to go to records presently outside of the form window. Finally use the record selector on the left edge of the form for selection. Then proceed with the execute-button in order to start data entry.
This form possesses a number of controls. Move with the cursor over the image. If the cursor changes from arrow to a hand, you can click to find detailed explanation about the controls.