Adobe really hosed CS3 and CS4 where this is concerned.
To open an animated gif with Photoshop CS3 or CS4 choose File > Import > Video Frames to Layers. This will open the animation but in my experience you lose all transparency for the frames. So you have to reconstruct the transparency if it exists. There is simply no way to open a transparent animated gif and retain both frames and transparency with Photoshop CS# or CS4.
It may be worth your while to go find some freeware which will open the gif and save it as a PSD. If I remember correctly, at the time, I found a $5 app that did just that and solved the issue. I can't recall the app name now though. I simply found it by searching for "edit Animated Gif" on download sites.
This issue was fixed in Photoshop CS5. Photoshop CS5+ will open transparent animated gifs without issue.