document.getElementsByClassName('btn-pageMenu') delivers a nodeList. You should use: document.getElementsByClassName('btn-pageMenu')[0].style.display (if it’s the first element from that list you want to change.
If you want to change style.display for all nodes loop through the list:
var elems = document.getElementsByClassName('btn-pageMenu');
for (var i=0;i<elems.length;i+=1){
elems[i].style.display = 'block';
}
to be complete: if you use jquery it is as simple as:
$('.btn-pageMenu').css('display','block');