which office version are you using ? in 2003 go to View-->toolbars-->forms
you will get a toolbar like this
now click the first checkbox to insert your check box, then click the small yellow lock at the end to lock the check-box so you will able to tick in it.
if you're on 2007 do the following :
go to your quick access toolbar like below, and click more commands
change the drop list box below
to
ALL COMMANDS like below
now browse until you see
insert form field and just add it to your toolbar.
now on your toolbar click the greenish circular thingy
it will ask you what you want to do,
just click checkbox and you're all set, the checkox will appear ,to CHECK it, just right-click it, go to properties, then at the default value, click
CHECK