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I can't answer 'exactly' since I don't have the Mediawiki module installed (and unforutnately the link to download it is currently non-operational).
To see if the MediaWiki module supports using the XOOPS search function look in /modules/mediawiki/include for a file called search.inc.php. If the file is there then the XOOPS search engine should be able to search the fields that the developers setup. If the file doesn't exist then XOOPS search won't 'crawl' the MediaWiki content.
It is, of course, possible to have the XOOPS search engine find content in MediaWiki but you may have to create or modify the search.inc.php file to make sure it searches the content you want. I'm guessing that you can find a reasonably priced developer to create/modify the file for you if you're unable to code php.
One other option is to contact the module developer and have them do it since it would seem to me to be a valuable "addition" to the MediaWiki module.