Good news, we just finalized a series of preloaded Seaside images on Squeak.org. This became necessary since builder.seaside.st is no longer accessible and consequently our instructions for installing Seaside in Squeak were out-of-date. The new images fix this problem and provide an easy install (for new users; this includes image, sources, vms etc. in an all-in-one setup) as well as an expert install (which just includes the images):
In addition, we now also provide 2.8 based images for people who have existing Seaside 2.8 applications in older versions in Squeak:
Here is a screenshot of the welcome screen:
Seaside 3.0 in Squeak 4.1
Oh, and if you want to load Seaside 2.8 / 3.0 into your custom Squeak variants; the above images include instructions about how they were created so you should be able to apply the same procedure to your own images.