You install them where you install other plug-ins, as described
here.
Which Pixel Bender plug-ins, exactly, are you trying to isntall and use? Where are you looking for them in the After Effects interface?
Note that you need to restart After Effects for it to load newly installed plug-ins.
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Todd Kopriva, Adobe Systems Incorporated
Technical Support for professional video software
After Effects Help & Support
Premiere Pro Help & Support
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