Try changing this line:

items.WithProgressReporting(Function(progress)
worker.ReportProgress(progress)).ForEach(Function(item) Thread.Sleep(10)) ' simulate some real work

To this:

items.WithProgressReporting(Function(progress) _
worker.ReportProgress(progress)).ToList.ForEach(Function(item) Thread.Sleep(10)) ' simulate some real work

The .ForEach extension method doesn't work with IEnumerables and since that's what WithProgressReporting returns, the resultset must first be cast as a Generic List. Also, I don't know for sure, but it looks like the converter inserted an extra line break. That's why I added the underscore to the end of the first line.