CiviVolunteer enabled but not showing in navigation
When a new Extension or Component is added you may find there are Permissions that need to be enabled, and that might be why you don't see it in the menu.

Alternatively go to Administer > Customise > Navigation Menu, check if it is in that list, if it is, but not showing, right click on it and 'save' and then you should see a new option on the screen that says "Click here to reload the page and see your changes in the menu bar above. " and that might be enough of a nudge.