You did not install class module (it is required unless you plan to write custom or use addon as framework)
You installed it incorrectly by placing it inside main framework addon -> learn how to install addons by hand
You have no idea how to properly install addons -> use curse client, it will work for sure.
2. No button is being circled
Reset maxdps settings in Interface -> Addons
Check errors in chat window, not every talent is supported
Check if your specialization is supported
Check if your action bar addon is supported
Aside from class module, main MaxDps Rotation Helper is required for modules to work.
If it still doesn't work and you get error messages in chat, try disabling addons until you find out which one is causing troubles. There is at least one confirmed incompatible addon that breaks functionality of MaxDps.
3. I'm lower level than 120 and it doesn't work/ find skills on action bars
Lower levels are not supported and will never be unless you write it yourself:
Far too much work
You don't really need dps helper while leveling
Hardly any mob doesn't die from few spells
4. "Original rotation" doesn't work when copied to custom rotations window
Don't even bother unless you got some real programming skills