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 Silhouette Studio. Please click here for a guide on how to install fonts to a Windows PC.
If you are using the free Basic Edition of Silhouette Studio you must use the PNG or DXF version of the file.
If you are using Designer Edition you may use the SVG version of the file.
Business Edition, you can use the SVG or EPS version of the file.
Last - but not least, if it is unzipped, is not a font file, and you are choosing the appropriate file extension for your version -- 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.