Whether you pull up the Term Store Management Tool from Central Administration or via Site Settings for the site collection the ability to add new terms in disabled unless you explicitly add your account to the service administrators in Central Administration. To enabled an account to manage the term store follow the steps below:
You are done. Please note that this dialog is empty by default, meaning no one really has any permissions to customize the term store until these permissions are granted.
Additionally you might have to specify the same account or accounts in the management tool itself under the node you wish to designate as follows: