Hi Hannes,
I see where you're going, and that's part of another way of installing. You want to ignore Seaside repository onsqueaksource.com unless you get a few things first. The way I'm describing wants you to go to squeaksource.com/MetacelloRepository. Then you see lots of ConfigurationOf*** files. Load the one you want. Open a Workspace. Type "ConfigurationOf*** load" then do it. Seaside3.0 takes easily seven minutes to load. Then in a workspace type WAKom startOn: 8080 and then do it. Then go to localhost:8080/seaside and you'll see the browse page.
I have a bunch of videos on the squeak.org site. I haven't looked at them in about two months:
http://www.squeak.org/Documentation/Installation/#h-5
They should have answers to all your questions including the method you tried first. If there's any problem with those videos or instructions, please let me know.
Chris