Had a bit of an issue with After Effects CS4 not showing older Trapcode plugins. After installation, the Trapcode folder was simply not present. Specifically, I had issues with SoundKeys 1.12, Particular 1.5, and 3D Stroke 2.04.

Once you install these plugins, they will install to your Program Files\Trapcode. Copy 3D Stroke and Particular *.aex files into Program Files\Adobe\Adobe After Effects CS4\Support Files\Plug-ins\Effects, and SoundKeys into the parent folder (Plug-ins). Restart After Effects.

For other Trapcode plugins, I haven't tried to install them yet, so I don't know.

I believe the newest versions of Trapcode plugins all install to the correct folder.

Update (2010-10-26): I'm probably late to the game on this, but older Trapcode plugins will not work in After Effects CS5. This version requires a 64-bit operating system and plugins, so you will now have to re-purchase your Trapcode plugins. Oh yeah, and Trapcode was bought by Red Giant, so you'll have to buy it from them.

I feel like Adobe just gave me the finger. I guess the assumption is that if you're working on video, you can afford it? Even with Photoshop I got a 32-bit version WITH the 64-bit (though I use the 64-bit one).