mypopmc tells Flash which movie clip you want to pull from your project library. (Use the Identifier name you specified earlier.)
window is the instance name you are assigning your movie clip once it is on the stage.
1 is the Flash Player level where you want to put the movie clip.
_x and _y tell Flash where to put the instance on the stage.
Note: Once you use ActionScript to put the movie on the stage, you can manipulate it like any other movie clip. You can add additional script to fade it, rotate it, change its color, etc. Just refer to it in your ActionScript using its instance name.
Test your movie. You should now be able to open your pop-up window using your button on the stage.