You use the Select method to set the cursor position in a Word document. Normally the Select method will be used to highlight a range of text, but if you use the same start and end positions in the Range property it just sets the cursor position.
Your code would look something like this:
using Word=Microsoft.Office.Interop.Word;
...
Word.Document wordDoc = .....
wordDoc.Range(10, 10).Select();
This will move the cursor to the 10th character in your document.