There's a good chance that the orientation of the blend door will not match that of the new actuator you bought. Since it isn't possible to rotate the servo without supplying a signal to it, you'll have to move the blend door to line up with the servo.

The white post is what the actuator turns. You should be able to turn this by hand to line up with your new actuator.

Now reinstall the new actuator, don't forget to connect the electrical connector.