Cannot log into My Little Wiki using MediaWiki Account.
http://mylittlewiki.org/w/index.php?title=Special:UserLogin&returnto=Main+Page
Topic on Project:Support desk
If by "MediaWiki Account" you meant an account created on mediawiki.org, then this wouldn't work as My Little Wiki is not connected to the Wikimedia foundation.
If by "MediaWiki Account" you meant an account created on My Little Wiki, you will have to contact that site's administrators. Contact information mentioned at that site's Main_Page and About pages.