I cannot find my repository.

SideCI may not access your GitHub organization.
This problem can be solved by granting SideCI access permission to your Organizations on GitHub as follows:

  1. In your account settings, proceed [Settings] => [Authorized OAuth Apps] and select SideCI.
  2. Find the organization where your organizations are listed in Organization access.
  3. Click Grant access.

I cannot add my repository.

Users who can add their repository must have admin permission for the repository. So, in case of this, contact admin users of the repository.

How can I change repository settings?

Sign in and select repository page you would like to change settings. And click [Project Settings] in left column. Then you can change your repository settings, which are selecting analysis tools, setting notification, switching modes and disabling the repository.
However in order to change the settings excluding disabling the repository, you must have write permission for your repository. You must have admin permission to make the repository disabled.