Sorry that add the device to the account via computer is not supported by AirDroid for the time being. If you want to add the device to the account, you have to sign in the account on the app in your device and then sign in the same account to web.airdroid.com.
You can check this link for more information. http://help.airdroid.com/customer/porta ... f-airdroid
You can check this link for more information. http://help.airdroid.com/customer/porta ... f-airdroid