$ sudo apt-get update
$ sudo apt-get install postgresql-9.3-plproxy 

postgresql-9.3-plproxy is installed in your system.

Make ensure the postgresql-9.3-plproxy package were installed using the commands given below,

$ sudo dpkg-query -l | grep postgresql-9.3-plproxy *

You will get with postgresql-9.3-plproxy package name, version, architecture and description in a table

postgresql-9.3-plproxy package basic information: