Some older versions of stuffit expander turn off the unix execute flag  
on the binary (for paranoid virus issues)
This of course prevents the program from running.
Which version of stuffit expander did you use?

To fix use a terminal session and cd down to the binary
So for example
~\Squeak\ 3.7.5Beta1.app/Contents/MacOS/

[ap0:Squeak 3.7.5Beta1.app/Contents/MacOS] johnmci% ls -l
total 2096
-rwxr-xr-x  1 johnmci  staff  1066560  1 Sep 19:01 Squeak VM Opt
-rw-r--r--  1 johnmci  staff       66  2 Sep 12:53 Squeak VM Opt.sig
[ap0:Squeak 3.7.5Beta1.app/Contents/MacOS] johnmci%


Note the "Squeak VM Opt" has x set

chmod a+x Squeak\ VM\ Opt

to set the x bit


On Sep 2, 2004, at 7:28 PM, Raymond Asselin wrote:

> Le 2004/09/02, John M McIntosh <johnmci at smalltalkconsulting.com>
> écrivait :
>
>> A beta version for testing can be found at
>>
>> see http://homepage.mac.com/johnmci/
>
> Hi John
>     in MacOSX  nothing happens when double clicking the
> Squeak 3.7.5Beta1.app or when dropping a Squeak.image on it...??
>
>
>
--
======================================================================== 
===
John M. McIntosh <johnmci at smalltalkconsulting.com> 1-800-477-2659
Corporate Smalltalk Consulting Ltd.  http://www.smalltalkconsulting.com
======================================================================== 
===