Most issues you experience when working with External Lists are due to permissions. Always check that you have the required permissions at both the data source and within the BCS service application. Some of the error messages within the browser are not very descriptive. When you troubleshoot issues in 2010 and you receive an error in the browser, always write down the Correlation ID. Using this ID, the administrator of your environment can use it to find more information in the SharePoint logs.