I have managed to create a solution to the problem.
You must call a function from an onclick function (e.g. onclick=
"selectall([name of selectbox])" of a checkbox. then you need to build a
function much like the one below:
function selectall(chosen)
{
var selObj = document.getElementById(chosen);
for (var i = 0; i < selObj.options.length; i++)
{
selObj.options.selected = true;
}
}