This backend version is not supported to design database diagrams or tables. (MS Visual Database Tools)

From previous I try to change table field size in SQL Server Management Studio I got this message appear and can't change anything with my table so I realize I use Server Management 2008R2 but my Database version is 2012 for now so I change Server Management to use version 2012 and it work fine. to avoid this don't use old version to edit new version that's all.

Where can I find Microsoft.SqlServer.Smo dll

Please check in %ProgramFiles%\Microsoft SQL Server\100\SDK\Assemblies

But you must install "Microsoft SQL Server 2008 Management Objects" first.

For Database
Microsoft.SqlServer.Smo
C:\Program Files\Microsoft SQL Server\100\SDK\Assemblies\Microsoft.SqlServer.Smo.dll

For ServerConnection
Microsoft.SqlServer.ConnectionInfo
C:\Program Files\Microsoft SQL Server\100\SDK\Assemblies\Microsoft.SqlServer.ConnectionInfo.dll

For Urn
Microsoft.SqlServer.Management.Sdk.Sfc;
C:\Program Files\Microsoft SQL Server\100\SDK\Assemblies\Microsoft.SqlServer.Management.Sdk.Sfc.dll