using DotNetNuke.Services.FileSystem;

// get instance of folder manager
IFolderManager folderManager = FolderManager.Instance;

// get an instance of folderInfo for thisUserInfo
IFolderInfo foldera = folderManager.GetUserFolder(thisUserInfo);

// get the folder path - relative to current portal root.
String userFolder = foldera.FolderPath;

// you can then also verify the folder is actually created
var folderb = folderManager.GetFolder(PortalId, userFolder);
String foundFolder  = folderb.FolderPath;

Westa