About the security manager 

The security manager is that part of the browser which controls what java applets can do on your computer. It prevents the Java applet doing harm to your files and your personal data. As a general rule, security managers are written in such a way that they do not allow the applet to read or write files but on the computer they come from.
For instance, if you view a web page at www.example.net containing an applet then this applet runs on your computer but can only read and write data on www.example.net, not on your computer. Thus your personal data are perfectly safe.

However, there are tiny differences in the security managers of Netscape and Microsoft concerning applets stored locally on your computer (such as kPlot if you have installed it locally). According to my experience, Netscape's Communicator then allows the applet to read and write on your computer. This allows kPlot to read locally stored animations.
On the other hand, Microsoft's Internet Explorer does not allow local applets to read or write on your computer. Thus, kPlot cannot run locally with this browser.