What have you done with the files which are supposed to be in the flexdock package?
Have you got the same folder containing your classes as the folder which contains the "com" folder?
You will have a hierarchy of folders, from
root--->classesFolder--->projectFolder--->com--->flexdock--->docking--->defaults.
Your classes ought to be where it will be obvious how to get to the "com" folder.
If your classes are in, say,
root--->classesFolder--->projectFolder--->com--->elango--->javaranch--->forum,
then they ought to have
"package com.elango.javaranch.forum;" in, and the "com" ought to refer to the same folder.
Are you using command prompt (if in Windows) or an
IDE?
If using command prompt you can put all the files in a single folder, navigate to it, then using the
javac -d . MyFile.java
command, both compile the file and create the correct directories simultaneously.
If com.flexdock is in a jar, there are ways or adding the jar to the classpath on command prompt, but I can't remember how to do it. It has been discussed on the Ranch before. You can probably find it with "search."
If you are using an IDE, then use the IDE's help to import the .jar.
Other people might be able to help more.