I have a Canon Powershot. On a recent trip, a friend downloaded his Olympus photos onto my SD card. While they don't show in my photos, I know they are in a folder on card. How do I access them?? When I plug into my Mac all I can see are my own pix, but not his and not the folder.
They should be there in a folder, probably within a folder called "DCIM." It would say "100OLYMP" or something like that. You'd go into the SD card volume just like going into a hard drive, via the Finder, and if they are on the card they should be plainly visible if you drill down into the folders.
If not, perhaps they got erased, and you could try some kind of recovery program, like CardRaider.
Maybe you can't see those pictures because folders (or files) won't show in the finder if their name start with a dot "." I use TinkerTool (http://www.bresink.com/osx/TinkerTool.html) to change that setting and see hidden files and folders.