#box1 h2
Boxheader=box1 h2;Boxcopy=box1 p
I might of misunderstood what you’re trying to do but your CSS rule would apply styles to a h2 element inside another element with an ID of box1.
The styles drop-down menu will only apply styles to an element that already exists. In which case, your CSS rule would have to be h2#box1.
If you want to wrap an element around the h2 i.e. a div, you would need to insert some predefined template code. There is a TinyMCE button for doing this, take a look at this thread to see if it helps.