You need to escape the slashes in your file paths, or else Lua will skip right over them when reading through.
Code:
button:SetNormalTexture("Interface\\Buttons\\UI-Panel-MinimizeButton-Up")
button:SetPushedTexture("Interface\\Buttons\\UI-Panel-MinimizeButton-Down")
button:SetHighlightTexture("Interface\\Buttons\\UI-Panel-MinimizeButton-Highlight", "ADD")