arcobjects - How to define Subroutines and functions?

I am working in VBA. My error is sub and function are not defined. Here is my code:

Private Sub CreateMap(cboDistrict As String)   
    Dim mapDocument As New mapDocument
    If mapDocument.IsPresent("K:\TASS\4_MAPPING_DATA_SUPPORT\Traffic_Mapping\District_Maps\2012\Abilene\Abilene_Base_Map.mxd") Then
        mapDocument.Open ("K:\TASS\4_MAPPING_DATA_SUPPORT\Traffic_Mapping\District_Maps\2012\Abilene\Abilene_Base_Map.mxd")
    End If
End Sub

I am trying to open an mxd when the Map button is pushed.