i had the same problem, heres what i did to fix it:
put the disc in the computer (i know this goes without saying...)
double click on the icon of The Sims 2 DVD, which should open a window with a black background, a large picture of The Sims 2 logo, two folders, and a few lines of instructions.
follow the instructions- drag the folder from the window to the Macintosh HD icon on your desktop. A small window should pop up that says "Copying Items to 'Macintosh HD' "
after this, click the icon within the new folder inside the Macintosh HD location.
good luck.