What is not working for you?
I was able to display a pushbutton in the grid\Sample\In Depth\DropdownGrid sample by changing the button in the Renderer constructor the same as you. If you wantto set More... as the text, you need to set the button.Text property.
Public Sub New(grid As GridControlBase, cellModel As GridCellModelBase)
MyBase.New(grid, cellModel)
Me.DisableTextBox = True
DropDownButton = New GridCellButton(Me)
DropDownButton.Text = "More..."
Me._grid = Nothing
End Sub ''New
If you want to enlarge the button, you set the ButtonBar size in the CellModel constructor.
Public Sub New(grid As GridModel)
MyBase.New(grid)
ButtonBarSize = New Size(60, 20)
End Sub ''New