Q: El Capitan  iMessage access?

What do you mean, exactly? The Messages app is a core app on your Mac. You sign in and verify the Apple ID you want signed into the app.

Need more information about what you are asking.

Cheers,

GB

If you are asking about access to Messages, again, you have provided no information. See if this support article helps:

Mac Basics: Messages helps you chat instantly - Apple Support

The app is called Messages. And Messages is a core app on your Mac.

Cheers,

GB