postgres generate uuid
uuid-ossp is a contrib module, so it isn't loaded into the server by default. You must load it into your database to use it.
For modern PostgreSQL versions (9.1 and newer) that's easy:
CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
but for 9.0 and below you must instead run the SQL script to load the extension. See the documentation for contrib modules in 8.4.
For Pg 9.1 and newer instead read the current contrib docs and CREATE EXTENSION. These features do not exist in 9.0 or older versions, like your 8.4.
If you're using a packaged version of PostgreSQL you might need to install a separate package containing the contrib modules and extensions. Search your package manager database for 'postgres' and 'contrib'.