I had this exact same problem. I think it stems from having another workspace already open referencing the project you want to add. I got it working by:
- shutdown xcode completely
- double-click the workspace to launch Xcode and open just that workspace
- drag and drop the project file from the Finder
Worked fine. I have many workspaces all referencing an openssl project, and it seems to work OK when you only have one workspace open at a time in Xcode.