The easiest way I can think of goes like this:
1) From your web panel selct "Add a Domain" and add the domain "wiki.examplw.com"
2) Wait for the domain to propagate (should display an empty directory in a browser when propagated).
3) From your Web panel, select goodies-> One-Click installs
4)Select MediaWiki, and follow the instructions. DH will install a basic MediaWiki system in the directory you select (use the wiki.example.com domain you set up in step 1)
5) wait for the database to be created and browse to your new, as of yet unprotected wiki.example.com site and make sure your wiki works.
6)From your web panel, select .htaccess/webdav and follow the prompts to password protect your wiki.yourdomain.com directory.
7) distribute the user name/password you established in step 6 to your "authorized" users.
you are good to go. Good Luck!