Your whole code should like this:
Option Explicit
Private Sub UIButtonControl1_Click()
' Call the Test method
Test()
End Sub
Public Sub Test()
Dim pGP as Object
Set pGP = CreateObject("esriGeoprocessing.GPDispatch.1")
On Error GoTo EH
pGP.AddToolbox ("D:\DRASTIC_MODEL.tbx")
pGP.MyModel
MsgBox pGP.GetMessages(), vbOkOnly, "Success"
Exit Sub
EH:
MsgBox pGP.GetMessages(), vbOkOnly, "Error"
End Sub
Let us know what message do you get?