EROR : stat failed on file /usr/local/pgsql/lib/plpgsql.so

I need some help in using plpgsql.
I use Postgresql 6.4 with a linux.
The instalation is complete, the regression test report ok on plpgsql,
I create a function (a trigger insert function) on a table in a database and
is also ok, but when I insert o record in that table

EROR : stat failed on file /usr/local/pgsql/lib/plpgsql.so

Were I am doing wrong? How can I solve the problem?

Thanks.

Octavian