and here's the source.
If you add the line
acl.write=/tmpto your ~/.hotjava/properties file, then the appletviewer will be able to write to that directory. That is, the appletviewer will be able to create files in your /tmp directory.
If you add a file name specifically,
acl.write=/tmp/foothen the appletviewer will be allowed to create just that one file, but not other files in the /tmp directory.
~/.hotjava/properties, if you don't want the appletviewer to allow applets to create files on your system.