This is a very common error.  Your Teamspeak 3 server doesn't have the proper permissions to allow third-party add-ons and widgets to access it, but most TS3 servers aren't configured to allow those permissions initially anyway!

You can change this within the Teamspeak 3 client fairly easily, though.  First you'll need to enable the Advanced Permissions System:

• Click 'Settings'.
• Click 'Options'.
• Click 'Application' on the left sidebar.
• Click the box next to 'Advanced Permissions System'.
• Click 'OK'.

Now that you've enabled Advanced Permissions, you need to actually add the permissions to your server!  This can be done on the 'Server Groups' tab:

• Click 'Permissions'.
• Click 'Server Groups'.
• Click 'Show names' from the dropdown near the bottom of the menu.
• Choose 'Guest' from the left column.
• Add the following permissions to the 'Virtual Server - Information' category:
• b_virtualserver_info_view
• b_virtualserver_channel_list
• b_virtualserver_client_list
• Add the following permissions to the 'Group - Information' category:
• b_virtualserver_servergroup_list
• b_virtualserver_channelgroup_list

Once you've done this, make sure that the permissions are allowed (by doubleclicking their names) and you should be set!