Just copy Microsoft.VisualStudio.SourceSafe.Interop.dll from:
C:\Program Files\Common Files\Microsoft Shared\MSEnv\PublicAssemblies
to
C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\PublicAssemblies
It then shows up in the macros ide in Visual Studio 2008, under Project | Add Reference...
Once you've added the reference, you can import it into your macro project and the Intellisense will show the constants e.g. for VSSFLAG_GETYES:
Imports Microsoft.VisualStudio.SourceSafe.Interop
...
sourceSafeFile.Get(csLocalFilePath, VSSFlags.VSSFLAG_GETYES + VSSFlags.VSSFLAG_TIMENOW)
Haven't tried creating an object using the interop DLL, but it probably also works.