The extension uuid-ossp is distributed with most newer versions of PostgreSQL. To confirm your PostgreSQL installation includes the extension, check for the uuid-ossp files in the following directory: install_directory\PostgreSQL\9.x\share\extension\.
Assuming the extension is installed, simply run the following SQL statement against your target database:
create extension if not exists "uuid-ossp";
You should add the entension to each database you use. Also add it to the template1 database, so that any future databases you create from template1 will inherit the extension.