Drop the tables created by MediaWiki 1.18 (drop & recreate the db). Import the old database in mysql. Run maintenance/update.php, so that MediaWiki 1.18 updates the db to the latest schema.
You will want $wgMysql5 = false; Beware of http://www.mediawiki.org/wiki/Manual:Backing_up_a_wiki#Latin-1_to_UTF-8_conv... you may need to add --default-character-set=latin1 to the mysqldump command.