Need to:

Hold down Alt & F11 to bring up VBA.

Add below code:

Sub InsertRowsAtIntervals()
'Updateby20150707
Dim Rng As Range
Dim xInterval As Integer
Dim xRows As Integer
Dim xRowsCount As Integer
Dim xNum1 As Integer
Dim xNum2 As Integer
Dim WorkRng As Range
Dim xWs As Worksheet
xTitleId = "KutoolsforExcel"
Set WorkRng = Application.Selection
Set WorkRng = Application.InputBox("Range", xTitleId, WorkRng.Address, Type:=8)
xRowsCount = WorkRng.Rows.Count
xInterval = Application.InputBox("Enter row interval. ", xTitleId, 1, Type:=1)
xRows = Application.InputBox("How many rows to insert at each interval? ", xTitleId, 1, Type:=1)
xNum1 = WorkRng.Row + xInterval
xNum2 = xRows + xInterval
Set xWs = WorkRng.Parent
For i = 1 To Int(xRowsCount / xInterval)
    xWs.Range(xWs.Cells(xNum1, WorkRng.Column), xWs.Cells(xNum1 + xRows - 1, WorkRng.Column)).Select
    Application.Selection.EntireRow.Insert
    xNum1 = xNum1 + xNum2
Next
End Sub

Press F5 to run code