SuperCard 3.0 provides a window property called "autoCenter" - when set to true the window is automatically drawn at the center of the main monitor when it is opened.
For earlier versions of SuperCard there are a number of ways to do this, but one of the fastest and cleanest ways is to simply center it before opening it, using SuperTalk's "screenLoc" function:
-- Sample "About" script
-- This script will set the loc of a window name "About"
-- match the center of the current monitor, and then open it.
-- Remember that a window's location is always given in
-- global coordinates.
set the loc of window "About" to screenLoc()
show wd "About"