See this page for information on installing and loading plug-ins.
What, exactly, are you doing when you get this error message? (I suspect that you're trying to import them as if they're source footage.)
Are you sure that you got the right versions of the plug-ins for CS4? The 64-bit version only works with CS5, and the 32-bit version only works with CS4.
It seems a little odd that you'd be loading these plug-ins as one of the first things that you'd do if new to After Effects.