Can you show us the code you're using?
It's also possible that you need to 'delay' your code until Stuf has actually created the player frame, using an OnEvent function. Preferably, wait until VARIABLES_LOADED has fired. Something like this:
Code:
local f = CreateFrame("Frame")
f:RegisterEvent("VARIABLES_LOADED")
f:SetScript("OnEvent", function()
--your code goes here
end)