Conditions are set on the left. (i.e when mouse clicked and variablex = 1) [note you can stack the conditions or even have sub conditions/sub events]
Actions are on the right (i.w set the Boolean true or false)
i dont think you can set a Boolean by expression in an action (you would do it in a separate condition)
If you want set the variable directly in the action then use a normal variable, if it is to be a Boolean type just fill it with a 1 or a 0
then you can evaluate something else to get the return value for the variable in the action.
edit
sorry i just read what I wrote and I cant even understand it.
to simplify
as far as i can see you cant set a Boolean with an expression in an action
but you could set the conditions then just have another event that checks the conditions and sets the boolean