do not use port 81. I have just had the same problem as you. Port 81 is used by SSL, so its not a great port to use for this. Much better to use something like port 2929, which isn't used by other apps already.
i'm finding that when you access the localhost page you have to manually key the page as localhost:port number, so it looks like: [
localhost] which should work.
dont forget to make the changes to that port in the apache config file first at the line where it says Listen 80.
good luck.