For the benefit of those folks who still find there way here via Google, below are updated instructions for enabling spell-check in Pidgin 2.7 and newer. Thanks to nascent for pointing out this change.
Shamelessly copied from the Pidgin FAQ
Versions 2.7.0 and newer use the dictionaries available from the openoffice download site. You need to download the appropriate language zip file(s) (files are in the form of two-character language code followed by a two-character country code - United States English is en_US.zip). Once you have downloaded the dictionary zip file, you need to extract it to
\spellcheck\share\enchant\myspell\(create the directories if they don't exist already). If Pidgin is running, it needs to be restarted for new dictionaries to be used.