Just one more comment in addition to kevbot's excellent post:

You are not allowed to write into directory "/Users/ad/.cpan/sources/authors"

This can also mean that a user other than ad has been working in that directory, for example if one has done sudo cpan in the past. If you still have the permissions problem after you've sorted out the confusion with the /Users/ad vs. /Users/fc, the simplest thing (although it'll cause a bit of re-downloading) is probably just to clobber the entire .cpan directory, making sure to back up the configuration file if you have set up a custom configuration. You may have to do sudo rm, but of course be very careful with this command!