In order to use the Pocket importer and to create Pocket subscriptions, you'll need to link your Pocket account to ReadCast.
Go to Settings
Under the 'Integrations' heading, click on the 'Connect to Pocket' button.
You will be redirected to Pocket to give ReadCast permission to access your account. Once accepted, you'll come back to ReadCast and you should get some green text saying your account has been linked.