Do you already have a designer repository installed. If not, follow the installation guide which comes with the Designer Documentation of the Developer Suite.
If you have a designer repository installed, did you create a new user in the designer repository database. After you created the user you can assign the user to the repository with the RAU.
Next time when you say you are having an error, please post the error. Give additional information like version of the Developer Suite, version of your database etc. The more information you share with us, the easier we can help you.