The easiest way to set a field watchpoint is to right-click on the field's name in the class outline and select Add Field Watchpoint from the popup menu. If you use this method of adding a watchpoint, the class name and field name will be filled out already for you.
Another way to set a field watchpoint is to select Breakpoint | Add Breakpoint | Add Field Breakpoint.
You may set a watchpoint for any field as long as the field is not a constant (ie. both static and final).