[Phanx]

That error means that you wrote the wrong function name in your RegisterEvent call, or have a scoping issue. To avoid scoping issues, I usually define all my function names first, and then the functions:

local Visibility, Update, Path, ForceUpdate, Enable, Disable

function Visibility(self, event)
    -- stuff
end

function Update(self, event)
    -- stuff
end

-- etc