After installing MozRepl, add one of the following blocks of code to your rc. The first will make the MozRepl server start at Conkeror's startup. The second will start the server immediately, and has the additional property of letting you easily customize the port it listens on.

user_pref('extensions.mozrepl.autoStart', true);

// Mozrepl
if (';1' in Cc) {
  let mozrepl = Cc[';1']
  if (! mozrepl.isActive())