Actually I'm using
sudo apt-get install bzr bzr-builddeb equivs
(though sudo apt-get install postfix seems to be doing the same thing) from the terminal windows, As per the iFolder instructions for installing Simias, but postfix is a dependency (I guess). Someone else seemed to have the same problem as I, but there was no resolution of the issue there.
What is happening is that a configuration screen opens up, with the configuration options, but I can't seem to select anything (and there is no cancel option).
Any idea what is going wrong? How can I fix it?