The repository administrator or a user with Full permission on a document can grant permissions on it from the Permissions tab of its repository property sheet. You can grant permissions on the repository root, folders, PowerDesigner models and model packages, and external application files, but not on individual model diagrams or objects.
Check out - Read permission. When you create a user, she is inserted into the Public group, which by default is granted Read permission on the repository root.
First check in (to create a document in the repository) or creating a folder - Write permission on the target location (the repository root or a folder).
The user who creates a repository item has, by default, Full permission over it, though this permission can be reduced by an administrator. Other permissions are cascaded down from the parent folder or root, so that if another user has Write permission on the parent folder, then he will have Write permission on the new item.
Subsequent check ins (to update an existing document) - Read permission on the target location and Write permission on the document (including, for PowerDesigner models, all impacted packages).