There is way to create an empty movie clip dynamically, and set the x and y for it at the same time, but until you understand how it works, leave that method alone.
You can position holder mcs like this:
Create a blank mc.
Call it holder. Set the instance name to holder.
Drag it onto the stage from the library.
Double click on the mc or right click and select edit in place.
Draw a rectangle the same width and height as the swf you’ll be loading.
Make sure you have the whole rectangle selected then press CTRL + I.
An info panel should appear - you’ll see a 3x3 grid, with a black square and a white square.
Make sure “top left” square is black. If not, click it. That sets the registration point of the rectangle (ie. where the x and y will be calculated from).
Now in the X box type 0 and in the Y box type 0.
The top left of the rectangle should now be aligned with the “cross” of the movie clip.
Now go back to the main time line - come out of editing the movie clip.
You should now see a rectangle the same dimensions as the swf you wish to load.
Move it to where you want, now the external swf should load into there.
