Have you tried putting them in the folder /Library/Fonts/ ? (This is in the Library folder at the top level of the drive that OS X is installed on.)
There are actually a multiplicity of Fonts folders.
There's /System/Library/Fonts, but Apple wants complete control of all things in /System and has decreed that all third-party changes go in /Library.
This is good, because then you can replace /System in a clean reinstall without harming your add-ons (like fonts). So putting the fonts in /System/Library/Fonts is not a good idea.
Finally, each user has a Library folder in their home, which is for things pertinant to only that user. If only you want to use those fonts but there are many users using the OS X machine, drop them into /Users/yourname/Library/Fonts.