To start using Mail.ru Login Button you should get Mail.ru Client ID  and Client Secret, then fill these in according wpDiscuz options in Dashboard >wpDiscuz > Settings > Social Login and Share admin page (Mail.ru section):

1.  Login to your Mail.ru account.

2. Navigate to Create an application.

3. Click on the “Create an application” button

4. Fill your Project name and All redirect_uri in according fields and click “Connect site”. The Valid OAuth Redirect URIs can be found in the Dashboard > wpDiscuz > Settings > Social Login and Share admin page (Mail.ru section).

5. You’ll be redirected to the Editing the application, where you’ll find the Client ID and Client Secret of the application.

6. Copy Mail.ru Client ID and Client Secret and paste in according fields of the Mail.ru Section in Dashboard > wpDiscuz > Settings > Social Login and Share admin page.