Why do I get the error message when I compile a Java program that imports bprolog.plc.Plc?

You have to make sure the environment variable classpath is set correctly. Add the following setting to autoexec.bat on Windows,

      set classpath=.;%BPDIR%\plc.jar    

and add the following line to .cshrc on Unix,

      set classpath=.:$BPDIR\plc.jar

In this way, classpath will be set automatically every time when your computer starts.