When you added transmission fluid, did you have the car running? The proper
procedure is to start the car and warm it up, set the park brake, and add
fluid until it reads between the marks on the dipstick. Also, make sure you
are on a level surface.
It sounds like you added fluid when the car was off and either you added too
much or not enough. Now, honestly, I don't know how much of a difference
that really makes, but I know for sure it shouldn't set off your Check
Engine light. It almost sounds like the tranny fluid is still too low. You
also have to make sure you are using the proper transmission fluid, which
should be Mercon or Mercon III (check your owners manual).
I would also take it and have the codes read for free as I doubt adding
tranny fluid will shut off the Check Engine light.