If you are certain it has been unzipped - it may be the wrong file type. If you are trying to use a font - you will need to install the font to your device, prior to using it in Design Space.
Please click here for a guide on how to install fonts to a Windows PC.
Last - but not least, if it is unzipped and is not a font file -- please make sure you are not looking inside of the MACOSX file. If your file name begins with ._ then it is a MACOSX file. These are useless files that are created when a folder is zipped on a Mac and unzipped on a Windows computer.
Please go back to your main folder, and you will see the regular files/folders listed below the MACSOX folder -- those are the ones you will want to choose.