If you're confident that the font is in the correct font folder, and that FontBook (or other font manager) shows it as being Enabled; and you've rebooted: then either the download was corrupt (unlikely if the font appears in Photoshop), or the font itself is sub-standard in some way.
Creating fonts is a very tricky and time consuming business, and many free fonts have all sorts of issues -- either related to their metadata and the way they interact with the OS, or in terms of the metrics, kerning, etc.