Finding Screen Coordinates

I’m working with Sandi’s additions to “mouseclick” on the “use existing” button for importing style sheets from a Word document. My question is this: how do I find out the screen coordinates of the button I want the script to click? Or is my script wrong? (it compiles, though) Here’s the pertinent snippet:
set theText to choose file with prompt “Please select the first file.” delay 4 MouseClick At {600, 225} with LocalCoordinates MouseClick At {750, 200} with LocalCoordinates delay 2 set text of text box 3 of page 1 to theText
Thanks in advance, Don