I have an account in Box.net but I can’t use the java applet to upload the file. It turns out that it is due to the GCJ java webplugin used by my firefox 3. This is the default in Kubuntu Hardy.
In order to enable it, I need to use the Sun Java plugin. To install, just type
%sudo apt-get install sun-java6-plugin
and then you need to switch the java plugin to use the Sun Java plugin. To do that, follow the procedures in this post.