C:\Program Files\Microsoft SQL Server\100\SDK\Assemblies is the correct folder location.
You need to add references to:
Microsoft.SqlServer.ConnectionInfo.dll
Microsoft.SqlServer.Smo.dll
Microsoft.SqlServer.Management.Sdk.Sfc.dll
Microsoft.SqlServer.SqlEnum.dll
(These are the minimum files that are required to build an SMO application)
Ref.: How to: Create a Visual C# SMO Project in Visual Studio .NET