If your classpath includes only that one jar, then only classes in that jar will be found -- so your application class won't be found. You need to use a path with multiple entries. On UNIX, use ":" to separate the entries; on Windows, use ";". You can use "." (dot) to mean the current directory. So, for example, something like
java -classpath jt400.jar:. IFSListFiles
By including the "." entry, I'm telling Java to look in the current directory for classes, as well as in that jar.