You don't need layer and textures. The button already has textures: <NormalTexture>, <PushedTexture>, and <HighlightTexture>
The "green placeholder" is the texture object without any texture.
My guess is that "Interface\AddOns\MyAddon\Art\ResButtonUp.blp" is a invalid path or "ResButtonUp.blp" is a invalid texture file.
Test your code with these textures:
Code:
<NormalTexture file="Interface\Buttons\UI-PlusButton-Up"/>
<PushedTexture file="Interface\Buttons\UI-PlusButton-Down"/>
<DisabledTexture file="Interface\Buttons\UI-PlusButton-Disabled"/>
<HighlightTexture file="Interface\Buttons\UI-PlusButton-Hilight" alphaMode="ADD"/>