You have to adjust the profile to use YOUR calendar instead of mine
Mute the phone while you have a calendar entry that marks you as beeing unavailable.
From the profile screen (which is the start screen of Tasker) choose
New -> State -> App -> Calendar Entry
Set Available to No, choose your main calendar by long-pressing on the entry field named Calendar. Then click Done.
Select "New Task", then select the plus sign in the left bottom corner,
Select Audio Settings -> Silent Mode and set Mode as On. Click Done.
Again choose Done, to return to the profile screen.
Note that there is a bug in which a declined event that shows you as busy will still activate this profile.