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

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

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

and add the following line to .cshrc in Unix,

      set classpath=.:$BPDIR\plc.jar

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