I found the answer by my self. and that is
for using Backup and restore class of SMO we need to 5 reffereces.
Go to Your Application and Right Click on References folder and select Add Reference.
Now Go to "Browse" Tab and browse the following path- C:\Program Files\Microsoft SQL Server\110\SDK\Assemblies
Now Select the following dlls
- Microsoft.SqlServer.ConnectionInfo.dll
- Microsoft.SqlServer.Management.Sdk.Sfc.dll
- Microsoft.SqlServer.Smo.dll
- Microsoft.SqlServer.SmoExtended.dll
- Microsoft.SqlServer.SqlEnum.dll
I have added only this (Microsoft.SqlServer.Smo.dll) reference first but for Backup and Restore classes we also need this (Microsoft.SqlServer.SmoExtended.dll) reference