Q: How can I add a device to my account?

Sorry, how do you add what to your devices?

In order for you to use Messages on the Mac, you would need to be signed into your own iCloud ID and then go into the Messages app on the Mac, and set up your preferences. You can create a separate User ID for the Mac that could have your iCloud account and iCloud info. This support atricle should help:

OS X Yosemite: Set up users on your Mac

Best of luck,

GB

Your question is not at all clear.  It might mean, for example:

• My husband has a MacBook.  How can I access his computer so I can send/receive iMessages from there?  (Answer: Have your husband set up a user account for you.)
• I already have access to my husband's MacBook.  I have an account on there with my Apple ID and password.  How do I set up the Messages app to access iMessage?  (Answer: See this article - Mac Basics: Messages helps you chat instantly - Apple Support  )