First of all you must cast "sender" which is just an object
to "ComboBox".
Then you must get "SelectedValue".
Finally you must get the files that are in the selected directory and use it as data source for the ListBox.
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
var comboBox = (ComboBox) sender;
var directoryInfo = comboBox.SelectedValue as DirectoryInfo;
if(directoryInfo == null) return;
var filenames = Directory.GetFiles(directoryInfo.FullName);
listBox1.DataSource = filenames;
var shortFilenames = filenames.Select(x => Path.GetFileName(x)).ToList();
listBox1.DataSource = shortFilenames;
}