What Daimon said to do should work. Can you confirm you put these lines under the line for ID="ToolbarField"?
Code:
<Element ID="Toolbar_WebstoreButton" X="1" Y="1" Width="1" Height="1" />
<Element ID="Toolbar_WebstoreButton_ButtonLabel" X="1" Y="1" Width="1" Height="1" />
<Element ID="Toolbar_WebstoreButton_ButtonLable_Store" X="1" Y="1" Width="1" Height="1" />
</Element>
<Element ID="ToolbarButton_RightSlot_6" X="1" Y="1" Width="1" Height="1" />
<Element ID="ToolbarButton_RightSlot_7" X="1" Y="1" Width="1" Height="1" />
If that's not working you could try making the images transparent. Unfortunately the buttons look to be on top of your toolbar. You might not be able to click through them even though they're invisible.
Here's the code for the webstore button. FileName should be a fully transparent tga. You can use the one from Black Vellum (under beta interfaces) if you're looking for one.
Code:
<Mapping ArtAssetID="toolbar_webstore_button_normal" FileName="blank.tga"></Mapping>
<Mapping ArtAssetID="toolbar_webstore_button_pressed" FileName="blank.tga"></Mapping>
<Mapping ArtAssetID="toolbar_webstore_button_rollover" FileName="blank.tga"></Mapping>