Checkboxes only work when the worksheet is protected for "fill-in." That or you'll have to write a nice piece of VBA code to do it when clicked. I'd recommend protecting the document.
Also make sure that you are using the checkbox from the Forms toolbar. Just putting a box in, don't cut it (but you probably already figured that one out...)
You can still check the box, but you have to double click it, then set it to checked. This is generally not feasible for an end-user and can get confusing.
Best use for these is a protected template. You don't have to set a password, just protect it and everything will "light up like Christmas"
cckens
"Not always my best shot, but I hit the target now and then"
-me