If you do not have an Apple ID, which is one of the accounts you can use to access Messages for Mac, click the silver "Create an Apple ID..." button to make one now.
Ọ bụrụ na ịnweghị Apple ID , nke bụ otu n'ime akụkọ ị nwere ike iji iji nweta Ozi maka Mac, pịa ọlaọcha "Mepụta Mpị ID ID" ka ịme otu ugbu a.