can't get pg_stat_statements to work

Ubuntu 11.04
Postgresql 8.4

I've installed postgresql-contrib:

# sudo apt-get install postgresql-contrib

I now have a directory:
/usr/share/postgresql/8.4/contrib
I've added a line in postgresql.conf:

shared_preload_libraries = 'pg_stat_statements'

and I connected to postgres using pgAdmin and inside a test database, I
ran the

/usr/share/postgresql/8.4/contrib/pg_stat_statements.sql

However, unless I comment out the new line in postgresql.conf, the
server won't start.

Could anyone tell me how to make this work?

Richard