The tutorial was written with Squeak in mind and should work for Seaside 3.
You can install Seaside in a Squeak image with the installation instructions from the Squeak Website (http://squeak.org/projects/#seaside).
The latest pre-built Squeak image with Seaside (Squeak 4.4 with Seaside 3.0.8) can be found on Squeak's file server at http://ftp.squeak.org/4.4/seaside/.
At the time of writing, a more current image based on Squeak 5 and Seaside 3.2 is being prepared.
[EDIT]
Execute the following in Squeak 4.5, 4.6, or 5.0 to get a current Metacello (taken from https://github.com/dalehenrich/metacello-work):
"Get the Metacello configuration (for Squeak users)"
Installer gemsource
project: 'metacello';
addPackage: 'ConfigurationOfMetacello';
install.
"Bootstrap Metacello Preview, using mcz files (#'previewBootstrap' symbolic version"
((Smalltalk at: #ConfigurationOfMetacello) project
version: #'previewBootstrap') load.
"Load the Preview version of Metacello from GitHub"
(Smalltalk at: #Metacello) new
configuration: 'MetacelloPreview';
version: #stable;
repository: 'github://dalehenrich/metacello-work:configuration';
load.
"Now load latest version of Metacello"
(Smalltalk at: #Metacello) new
baseline: 'Metacello';
repository: 'github://dalehenrich/metacello-work:master/repository';
get.
(Smalltalk at: #Metacello) new
baseline: 'Metacello';
repository: 'github://dalehenrich/metacello-work:master/repository';
load.
Then, the following will install Seaside+Magritte
Metacello new
configuration: 'Seaside3';
repository: 'http://www.smalltalkhub.com/mc/Seaside/MetacelloConfigurations/main';
version: #stable;
load: 'OneClick'.
Metacello new
configuration: 'Magritte3';
repository: 'http://www.smalltalkhub.com/mc/Magritte/Magritte3/main';
version: #stable;
load: 'Magritte-Seaside'.