If you just want to show textures and/or font strings on mouseover (like the highlight texture on buttons in default UI) you don't need to use scripts; you can just put them on the "HIGHLIGHT" layer:
Code:
-- A transparent yellow texture will appear on mouseover:
local highlightTexture = frame:CreateTexture(nil, "HIGHLIGHT")
highlightTexture:SetAllPoints(true)
highlightTexture:SetTexture(1, 1, 0, 0.5)
-- The word "Mouse!" will appear on mouseover:
local highlightFontString = frame:CreateFontString(nil, "HIGHLIGHT", "GameFontNormalLarge")
highlightFontString:SetPoint("CENTER")
highlightFontString:SetText("Mouse!")