Question: how to install zxp file in after effects?
Answer: Rename your .zxp extension file to use .zip instead. ( You'll get a warning, but just click OK)
Unzip the file, you should get a folder. ... 
Copy that folder into C:\Program Files (x86)\Common Files\Adobe\CEP\extensions\
Restart After Effects.
Your extension should now show up under Window > Extensions.