postfix failed to install under ubuntu

[Peter]

Hello,
I have ubuntu 6.1 64 installed.
  apt-get install postfix 
will failed with the error message:
  Failed to fetch http://us.archive.ubuntu.com/ubuntu/pool/main/p/postfix/postfix_2.3.3-1_amd64.deb  404 Not Found [IP: 91.189.88.46 80]

I searched the internet and find the package to install:
 http://apt.wikimedia.org/ubuntu/pool/main/p/postfix/postfix_2.3.3-1_amd64.deb

running the command  "dpkg -i postfix_2.3.3-1_amd64.deb", it prompts a text screen with "OK" button, but there is no way to move the mouse to OK button to click, typing "Enter" is useless as well.
I have to kill the process eventually. 
(note: I use ssh to login to install).

any suggestions to do it correctly?
thanks.

Peter