You need to store the value somewhere, or read it directly from the document e.g.
Code:
Private Sub CommandButton1_Click()
ActiveDocument.Variables("varMultiDef").value = ChxMultiDef.value
Unload Me
End Sub
Private Sub UserForm_Initialize()
Dim oVar As Variable
For Each oVar In ActiveDocument.Variables
Select Case oVar.Name
Case Is = "varMultiDef"
ChxMultiDef.value = oVar.value
Case Else
End Select
Next oVar
End Sub