Business Data Connectivity Service allow to connect with external data system (WCF service, database, .Net assemblies), which is improved version of formal Business Data Catalog in SharePoint 2007.
But you can get this type of error after installing SharePoint 2010 Beta and SharePoint Designer 2010, opening Designer and navigating to the External Content Types object results in the following error:
This problem can happen If your SharePoint installed in Windows 7 OS due to the following patch of Client OS (Windows 7) though another reason of this error is BDC service isn’t running properly. So, You need to install Windows6.1-KB976462-v2-x64 Hot fix from
http://connect.microsoft.com/VisualStudio/Downloads/DownloadDetails.aspx?DownloadID=23806
For Windows Server 2008, ensure all update are installed using Microsoft Windows Update. If the the problem persist again after restart PC then go to Manage services on server on Central Administration check BDC and Metadata service
Central Administration –> System Settings –> Manage services on server
Check if Business Data Connectivity Service and Managed Metadata Web Service are stopped. Start the services if any of them are stopped and close SharePoint Designer and re-open again.
The error should be resolved after that.
If BDC Metadata store is unavailable this try http://brianhochgurtel.blogspot.com/2009/12/bdc-metadata-store-is-currently.html