This extension has been removed, as the craigslist.org terms have changed to forbid users of Craigslist from providing scripts that interact with Craigslist. Specifically, the terms now state,
You agree not to use or provide software (except for general purpose web browsers and email clients, or software expressly licensed by us) or services that interact or interoperate with CL, e.g. for … flagging…. … [S]cripts …, etc. are prohibited.
For more information, see the cease and desist letter.