Your syntax is wrong. Depending on the type of combobox it would be one of the two:

Sub Example1()

'ActiveX
With ActiveWorkbook.Worksheets("yer maw").OLEObjects("ComboBox1").Object
    .Clear
    .AddItem "All Decks"
    .AddItem "maw 1"
    .AddItem "ma maw 2"
    .AddItem "his maw 3"
    .AddItem "Deck 4"
    .AddItem "Deck rrr"
End With

'Form Control
With ActiveWorkbook.Worksheets("yer maw").Shapes("Drop Down 1").ControlFormat
    .RemoveAllItems
    .AddItem "All Decks"
    .AddItem "maw 1"
    .AddItem "ma maw 2"
    .AddItem "his maw 3"
    .AddItem "Deck 4"
    .AddItem "Deck rrr"
End With

End Sub