Introduction

There is a doc on the Scribus web site, but it is partly outdated for current releases. The fastest way to get Scribus on the Mac is the following:

Installation

Install the Xcode developer tools from http://developer.apple.com/technology/xcode.html
Install Apple's X11 (two packages, X11User.pkg from the system disk and X11SDK.pkg from the developer tools disk, the names may be slightly different). Note: The Xcode developer tools may install the X11 packages. In this case you can skip this step.
Install Fink using the "Fink-X.Y.Z-ABC-Installer.dmg" from http://fink.sourceforge.net (where X.Y.Z is the latest stable version and ABC is is either PowerPC or Intel depending on your system)
For Version 1.3.3 - Open a terminal window then type:

fink install scribus

For Version 1.3.9 - Open a terminal window
Enable unstable packages (accept all default options --by pressing enter-- except to enable the "Unstable" tree):

fink configure

fink selfupdate-rsync

Install 1.3.9

fink install scribus135-aqua (yes, there is a version discrepancy in the fink package where 135-aqua is 1.3.9 at the moment)

Optional: type the following to see a list of scribus versions (as this article may not be updated often enough to get new versions):

fink list scribus

When the list is displayed you can alternatively type:

fink install <name of package from the list>

fink may prompt you to select certain packages based on dependencies it needs to install scribus. Accept any of the choices and continue.
Sit back, drink some coffee and wait for the installation to finish.
Once installation is complete you can run scribus by opening a terminal window and typing:

scribus