Opening an Eclipse project already in my workspace

If you are using Eclipse for Mac, you have to create a new project with the same name as the folder that contains the project, which is already in the workspace. Do nothing else (no imports), just finish the wizard, and you will find all the subfolders and files in the new project created.

It's very misleading though, I had the same problem and it got me half an hour to solve it.

It is already open, but your windows "Package explorer" and "Project explorer" are closed and you do not see it. Go to Tab Window then Show view and chose any of them.