If you mean that in the AdminCP, you do not have a big "Wiki" link down the left side, between "Content", "Forums", "Users", "Appearance", etc:
If you don't see this, then it is possible you performed the installation while not being a Super Administrator. Your forum's super administrator will need to grant you permission to manage VaultWiki.
Groups & permissions > Staff > Administrators > [your name]. While editing your name, there will be a section called Permissions. There is a setting here called "Manage VaultWiki" which you will need to have checked.
If you mean that in the public forums, you do not have a "Wiki" tab in your forum navigation:
Then you may not have permissions to view the wiki yet (you should already have this as admin).
Check AdminCP > Wiki > Permissions > Usergroup permissions.
The navigation tab might be disabled for some reason:
Check AdminCP > Setup > Navigation > Public navigation, looking for "Wiki" related entries.
You might also have custom navigation through specific style changes, or you might have a third-party add-on managing your navigation. In either of these cases, you would have to adjust your style changes / add-on settings accordingly which is beyond the scope of our support here.