see above, you should add using Ice.Lib.
This code works fine for me from BPM
var baseFolderType = Epicor.ServiceModel.Utilities.SpecialFolder.UserData;
var userDataPath = Ice.Lib.PathHelper.GetFolderPath(Epicor.ServiceModel.Utilities.SpecialFolder.UserData);
var absoluteFileName = System.IO.Path.Join(userDataPath, "subfoldername/anothername/file.ext");
throw new Exception(absoluteFileName);
It throws full path:
C:\EpicorData\Users\MANAGER\subfoldername/anothername/file.ext