To use an external Viewer, your TeamSpeak 3 server has to be appropriately configured to allow external information requests.
Follow the two following steps to do so.
Enable advanced permissions
First, you need to enable the advanced permissions in your client. Else you won't be allowed to add the required permissions to Viewer.
Navigate to the TS3 options of your client and tick the corresponding choice like below :
Add permissions to TS3 Viewer
In the permissions management menu of your TS3 server, proceed like below :
The permissions to add are :
Virtual Server -> Information:
b_virtualserver_info_view b_virtualserver_channel_list b_virtualserver_client_list
b_virtualserver_servergroup_list b_virtualserver_channelgroup_list