Here's a sample code snippet:
Excel.Worksheet sht = (Excel.Worksheet)ActiveSheet;
sht.Cells[3, 3] = "HELLO";
You can also capture ranges:
Excel.Range rng = (Excel.Range)sht.Cells[3, 3];
I believe that you are just the Select method, as before, to select a range, although I have not tested this.
rng.Select();
You can obviously streamline this and combine these statements with proper casting. I do not want to risk guessing here, because I do not have a VSTO project open from me.
EDIT
, get_Range:
rng = sht.get_Range("A1", Type.Missing);
VSTO , , get_Range . - , VSTO ( VBA , Excel).