Valid connection strings in Schema Explorer depend on the provider you're using for a given connection:
If you're using the SqlSchemaProvider, connection strings follow the format used by the .NET SqlConnection.ConnectionString property.
If you're using the ADOXSchemaProvider, connection strings follow the format used by the ADO ConnectionString property.
If you're using the OracleSchemaProvider, please be sure to check out the link below for creating a connection string as well as the following how-to article.
If you're using the PostgreSchemaProvider, please make sure the following statement is included in your ConnectionString: Preload Reader = true;
For all SchemaProviders, please be sure to use the proper ConnectionString for the Database Provider you are using. A great resource for building database ConnectionStrings can be found here.