Hi Oj,
Thank you for your question. If you have exchanged messages with a freelancer through oDesk Message Center, you can go to your Messages Tab, then access the message thread with that freelancer. On the next page you should see all the messages and a big green button "Hire."
Note though that you will only be able to hire a freelancer once you complete your client profile with your company details and a valid payment method.
I hope this information helps.
~ Valeria
