What kind of client are you using?
The zip file should be saved under the scratch folder of the arcgisjob, not scratch gdb. That's why you cannot obtain the zip file from the Viewer. Check your model, make sure the output is written to %scratchworkspace%\output.zip, not %scratchgdb%\output.zip.
I checked my model, it has %scratchworkspace%\output.zip
When I run extract widget in my application (flex) and after that I check the arcgisjob directory, there is just an output.zip file in my scratch.gdb folder and original scratch folder has not any output.zip!!!!!, but it has 3 subfolders(scratch.gdb, scratchFolder and ZipFolder).
What is wrong?