As an administrator, edit the wiki page “MediaWiki:Sidebar”. (The editinterface permission needs to be enabled and is enabled for administrators by default; for information on assigning it to other groups, see Manual:User rights.) You may need to use the syntax //yourdomain/yourwiki/index.php?title=MediaWiki:Sidebar&action=edit if you use long URLs.
If you are sure you are logged in with the relevant rights but do not see the edit link, click on the create tab at the top of the page – this will take you to the edit page.
Example sidebar code:
Sidebar example
* navigation
** mainpage|mainpage
** Special:Recentchanges|Recent changes
* new heading
** portal-url|portal
** http://www.mediawiki.org|MediaWiki home

