What I do is create my own menubar and toolbar and customize it. Then I don't have to worry about adding items at startup. However, sometimes it is necessary. So here's some code on how to do it. Note that you will have to set a Reference to "Microsoft Office 9.0 Object Library" if you want to be able to declare items as CommandBarControls. Else just declare the variable as an Object.
The following code adds a combobox to the main menu. The public function "NewItem_OnClick" is called when the user selects an item.
Dim cbcControl As CommandBarControl
On Error Resume Next
Set cbcControl = CommandBars("Menu Bar"

.Controls("NewItem"
If (Err.Number = 5) Then 'IFT, "New Item"
On Error GoTo ErrHandler
Set cbcControl = CommandBars("Menu Bar"

.Controls.Add(msoControlComboBox) '(msoControlButton)
With cbcControl
.Caption = "Show:"
.Tag = "NewItem"
.OnAction = "NewItem_OnClick"
.Visible = True
.Style = msoComboLabel
.BeginGroup = True
.DescriptionText = "Select Item"
End With
End If
Public Function NewItem_OnClick()
Dim cmbBar As CommandBarControl
Set cmbBar = CommandBars("Menu Bar"

.Controls("NewItem"
Msgbox cmbBar.Text
End Function