You need to install the package postgresql-contrib-9.1 in your system first. (Adapt to your version number! Here is the currently available list of packages.) That’s the case under Debian, Ubuntu & friends anyway. Using a system user with the necessary privileges:
apt-get install postgresql-contrib-9.1
If your currently logged in user does not have the necessary privileges (but sudo rights):
sudo apt-get install postgresql-contrib-9.1
Quoting the PostgreSQL Apt Repository:
If the version included in your version of Debian is not the one you
want, you can use the PostgreSQL Apt Repository.
There are basic install instructions on the Postgres website for each available OS.
For accent-insensitive indices using unaccent consider this related question:
- Does PostgreSQL support “accent insensitive” collations?