Jerry, what version of LR are you using? In 1.3.1, have you gone into the Preferences settings and set CS2 as an external editor? Also, have you set your external naming convention in there too?
If from CS2 you save the file with a new name from what it went over from LR as then LR will not find it. For example, if the file left LR name myfile_edit.tif and from CS2 you saved it as myfile_editedandcropped.tif then LR will not see it automatically.
To have LR pick up the file, you can go the Library Module in LR. Select the folder the file is saved in (assuming you saved the edited file into the same folder as the original), right click and select Synchronize Folder... This will find any new files in that folder and bring them into LR. You will then have to stack the original and edited files together yourself.
I recommend setting up a naming convention that works and trying to stay with it as much as possible.
Good luck,
Beau