First you will need username and password to your PostgreSQL cartridge. You can get them when you run rhc apps
Next run: rhc port-forward -a APP_NAME This will bind remote PostgreSQL instance to localhost and port 5432 (btw. if you already run PostgreSQL on your computer, it will automatically change that port to 5433).
Note: Currently make sure you use Ruby 1.9.x, otherwise you will get hard-to-debug errors.