Smith Micro support says that they don't support silent installation of StuffIt Expander. HA! Here is how to do it.
Download StuffIt Expander from www.stuffit.com
Download Microsoft Visual C++ Redistributable SP1 from Microsoft: http://www.microsoft.com/download/en/details.aspx?id=5582
(what you need from Microsoft is vcredist_x86.exe which is 4MB.
StuffIt Expander has a prerequisite that is Microsoft Visual C++ Redistributable SP1 that is downloaded when you run the .exe, which is why we need to download it from Microsoft.
Now we need to extract the .msi from the StuffItExpander.exe. Run StuffItExpander.exe and when you are at the first installation prompt stop and search your c: drive for stuffitexpander2011.msi. You'll find it in your temp folder. Save the stuffitexpander2011.msi
First step is to install the Visual C++ and you can do this silently with the following command line:
Vcredist_x86.exe /q:a /c:"msiexec /i vcredist.msi /qn /l*v %temp%\vcredist_x86.log"
After the Visual C++ is installed you can now install the StuffItExpander2011.msi with the typical silent switches:
msiexec.exe -i stuffitexpander2011.msi /qn
If you are deploying with SCCM of the like you can make the Visual C++ a prereq by telling it to run first.