I am not sure where you are doing this work, but you can position the cursor in an actively editing gridcell by getting the embedded textbox and using it to set the cursor position. Here is code that should put the cursor at the end of the text. (You can move it where ever you need it to be.)
GridTextBoxControl tb = this.grid.CurrentCell.Renderer.Control as GridTextBoxControl;
if(tb != null)
{
tb.SelectionStart = tb.TextLength;
tb.SelectionLength = 0;
}