Dynamic PDF

I have a form with 4 checkboxes ("Reason") acting as radio buttons (Deliver, Coordination, Efforts, Other).  When Other is selected I would like a text box to appear.  This is the script I am using:

reason= this.getField("Reason").value;
field = event.target;

if (reason == "Other") {
field.display = display.visable;
} else {
field.display = display.hidden;
field.value = "";
}

However, my text box is not appearing.  What am I doing wrong? 

I am new to writing javascript.

Thank you