Indeed, the Workbook and Worksheet classes are located in the Telerik.Windows.Documents.Spreadsheet.Model namespace. The ViewState property of Worksheet is introduced in the Q1 2015 release and if you are using an earlier version of the controls, you could update them in order to take advantage of this feature.
In order to use the Freeze Panes functionality with previous versions, you could follow the steps below:
1. Get the view state of the worksheet:
WorksheetViewStateworksheetViewState = (WorksheetViewState)((ISheet)workbook.ActiveWorksheet).ViewState;
2. Create a new pane and set it to the pane property of the view state:
Panepane = new Pane(new CellIndex(17, 4), 3, 5, ViewportPaneType.Scrollable);
worksheetViewState.Pane = pane;
Hope this helps.