At the location where you want the check boxes to appear, from the Insert
menu in Word, select Symbols and select the Wingdings 1 font and insert the
168 symbol (an empty box) then the Wingdings 254 symbol (a box containing a
tick mark).  Then insert a space between the two symbols and then select
them both and Press Ctrl+F9 so that they are enclosed inside field
delimiters { }.  Now, locate the selection after the opening { and type IF,
then insert the mergefield that contains the -1 or the 0 values and after it
type a space, then = -1 with a space after the minus one.

You should now have

{ IF <<mergefield>> = 0 symbol symbol }

Press Alt+F9 a couple of times to hide the display of the field codes and
then execute your merge.  You may need to use Ctrl+A to select everything in
the document that is created (assuming you execute the merge to a new
document) then press F9 to update the field codes in the document.