If you moved from Flash MX 2004 pro to Flash 8 and then opened an existing fla file that included remoting (or other classes) you might find that the compiler complains about missing classes when you try to test or export the movie. At first I assumed this was because I needed to run the flash remoting component file (an add-on that installs the classes) that I originally downloaded from Macromedia. That did not work because it tried to install the files in the "old" directory for the old installation of flash (mx 2004).
To fix the problem I had to go into:
Incidentally, I abhor the file structure of macromedia products. I would love to have "c:\mm\flash\configuration\...". They always tout how extensible everything is - and it is, if you can find it. If you know how I could have avoided this extra step please enlighten me!