I can't use my unzipped font file

When fonts are in a zip archive, typically they were created for use on a Windows machine. Windows fonts have a different format from Macintosh fonts. Therefore, you will need to convert the file you have unzipped from a Windows font to a Macintosh font, after you have unzipped it, before you can use it.

If the font is a TrueType font, you can use the utility TransType, available from FontLab to convert them to Macintosh format. (TransType is not related to ZipIt or supported by the author of ZipIt in any way.)