When selecting multiple values in a multiple choice field in the web app, you must use the standard shift-click or control-click (command-click on Mac) method.

Using the shift-click method will select all the values between the two clicks. 

To do this, hold the shift key and then click on the values that you wish to select. Once you are finished selecting the values you can release the shift key.

Using the control-click (command-click on Mac) will select just the values that are clicked on.

To do this, hold the control (command) key and then clikc on the values that you wish to select. Once you are finished selecting the values you can release the control (command) key.