To create a directory via scripting, first create a directory object. This can be performed using the keyword "new". The directory object can be obtained using the keyword "Dir" (see also the Trolltech language specification on the Dir object).
var dirobj = new Dir;
Then, use the Dir object's member function mkdir() to create a new directory:
dirobj.mkdir(dirname);
function Create_directory() {
var dirobj = new Dir;
var dirname = Input.getText("Please enter a directory name", "C:/", "Create directory");
dirobj.mkdir(dirname);
}