Usa la función Randomize para genera una semilla basada en la hora actual:
Código vb:
Ver originalPrivate Sub Command2_Click()
Dim a As Integer
Dim i As Integer
i = 0
Randomize()
Do
m = (Int((49 * Rnd) + 1))
If (chk0(m).Value <> vbChecked) Then
chk0(m).Value = vbChecked
chk0(m).BackColor = vbGreen
i = i + 1
End If
Loop Until (i = 6)