\n is newline, so replacing \n\n with \n should work.
Unless they also use carriage returns, in which case you'll be wanting something more like \r\n\r\n with \n (or \r\n if necessary)
However, when you say "to use in the Search and Replace box" you have me slightly confused - which search and replace box?
Not all of them do regex.
If you don't already have something that does it, then Metapad (
http://liquidninja.com/metapad/) has limited regex, and will work for this (it also avoids the carriage return issue too).