Try the last set field step without the = sign and see if you then get it to work.
When I do that I get an error saying (still within the specify calculation wondow) "An operator is expected here." :-/
That tells me you have the set field step set up incorrectly. (I thought it was a typo in your post originally.)
Click the first specify button and select checkbox_donotannounce from the Show table occurrence.
Click the second specify button and enter "Do Not Announce"
show::checkbox_donotannounce = "Do Not Announce"
Is a boolean expression that will, in find mode, always be False--which means it returns 0 in this case. Since you did not specify a target field, no value at all gets entered here.