Sorry, it was not appropriate as FAQ. Here is the code for you:
Sub Tigrek_MirrorSelectionSet()
'
' This example mirrors all entities in a selection set
' made by picking on screen
' I wrote this code today and tested a dozen times
' but there may still be bugs - would appreciate feedback posts here
Dim ssTigrek As AcadSelectionSet
Set ssTigrek = ThisDrawing.SelectionSets.Add("TIGREK_SSET1"

' Add entities to a selection set by prompting user to select on the
screen
ssTigrek.SelectOnScreen
'get the points for mirror axis:
Dim point1 As Variant
point1 = ThisDrawing.Utility.GetPoint(, "Enter first point of mirror
axis: "

Dim point2 As Variant
point2 = ThisDrawing.Utility.GetPoint(point1, "Enter second point: "

Dim mirrorObj As AcadEntity
For I = 0 To ssTigrek.Count - 1
Set mirrorObj = ssTigrek(I).Mirror(point1, point2)
Next
ThisDrawing.SelectionSets("TIGREK_SSET1"

.Delete
End Sub