Hi Andre!

Unfortunately there is no standard widget kit in Squeak, and therefore no
common UI Builder as you
know from commercial systems - although lots of people ask for it. 

You can either look at the new GTK+ bindings Goran described or have a look
at Zurgle from Jim Benson
(if you want a more Windows like Look & Feel)
http://minnow.cc.gatech.edu/squeak/2469

The nice thing on Squeak is that you are able to build own user interfaces
which are more understandable
for a user than most standard GUI's from operating systems. Have a look at
the Screenshost from the
Exobox projekt: http://minnow.cc.gatech.edu/squeak/1794
(unfortunately you dont get the source code for this)

Bye
Torsten