Try:
Sub MeanSDSEM()
x = InputBox("Enter number of samples")
ActiveCell.FormulaR1C1 = "=AVERAGE(RC[-1]:R[" & x - 1 & "]C[-1])"
ActiveCell.Offset(0, 1).Select
ActiveCell.FormulaR1C1 = "=STDEV(RC[-2]:R[" & x - 1 & "]C[-2])"
ActiveCell.Offset(0, 1).Select
ActiveCell.FormulaR1C1 = "=RC[-1]/SQRT(" & x & ")"
ActiveCell.Offset(1, 0).Select
End Sub