Have a look at the attached screenshot of a view I created to show three areas at once.
The upper-right quadrant is the Panes area - only one column is shown in the screenshot.
The lower-right is the File List area, where only one column (Dance Style) is shown.
The lower-left is the Action Window's Tag area. You can see the field named Dance Style.
In the File List, you can see that when a list type field is shown, the values for the field are listed in one row, separated by semicolons (eg. Balboa, East Coast, etc.). You can see also that two tracks are selected.
In the Panes area, the shown Dance Style pane shows the checkbox list of the values currently existing for the field. When there is a checkmark, it means the value is set for the selected tracks. When the box is blue, it means some of the selected tracks have the value set, some do not. An empty box of course means that value is not set. Compare the value "Balboa" in both the Dance Style column in the Panes view area and the Dance Style column in the File List area. The selected tracks have both the Balboa and Swing value set, but only the first tracks has Lindy, Charleston, etc.
In the File List area, select one or more tracks by selecting in the Dance Style column and then press F2 - a list edit will open where you can select values. Hit Enter to accept the values (there are other ways too).
To edit the values in a Pane, click an empty area in the Dance Style pane (not a value, or that changes the selection) and use F4 to enable tagging mode. You'll see the boxes change to editable.
In the Tag Action Window, show the Dance Style field (from the upper-left area of the Tag Action Window, either show all fields, or add this field to the default view) and click to Edit, add your checks, and either Close the edit box, or continue to edit other tracks with the box left open.