OnUpdate handlers are only called when the frame is visible.
OnUpdate handles are called every frame. Use with care.
When nesting frames, you can access the parent frame name in XML names with $parent, this is a textual substitution and cannot be used in Lua. The Lua equivalent to $parent is the GetParent() method of the frame (e.g. getglobal(Frame:GetParent():GetName().."Button") is the same as $parentButton).
All Frames have a GetID() function which will obtain the ID specified in the XML. ID's must be positive integers enclosed in quotes: ie. id="1" or id="20"
If you don't specify parent="UIParent" in your top-level frames, your frames won't be scaled according to the currently active UI scale. This can lead to some hair-pulling as you try to figure out why your fonts are so huge. (But, in some cases, not having that scaling might be desirable.)
If you get no error output, but your addon is simply not being loaded, make sure you don't have a syntax error in your .xml file. A simple way to ensure that your xml is well-formed is to open it in Firefox; it will point out any XML structure errors to you. You can also check the contents of your Logs directory for errors.
Output to the outside world
The only file output allowed is when the UI engine saves variables before it exits or reloads. Addons use the "## SavedVariables:" header to indicate which data they wish to save.
x,y = GetCursorPosition(); will return the current position of the mouse with respect to the object it currently is in. This means x will be 0 if the mouse is at the left edge of a frame or button, regardless of where that frame or button is.
x,y = GetCursorPosition(UIParent); will return the current position of the mouse with respect to the entire UI. This can be considered the absolute position..