By default concrete5 doesn't have .zip extension in the list of allowed file types. To change that, go to "File manager" -> "Access" tab. There you will see the "Allowed File Types" panel, which has these file extensions by default:

flv, jpg, gif, jpeg, ico, docx, xla, png, psd, swf, doc, txt, xls, csv, pdf, tiff, rtf, m4a, mov, wmv, mpeg, mpg, wav, avi, mp4, mp3, qt, ppt, kml, xml

Add zip and other file extensions you want to be allowed for upload and click "Save".