Download the plugin from GitHub and copy the yuml directory to /lib/plugins of your DokuWiki installation.
You can put a Yuml.me class diagram of use case diagram in your page with following syntax:
<classdiagram>
[Customer]+1→*[Order]
[Order]++1-items >*[LineItem]
[Order]-0..1>[PaymentMethod]
</classdiagram>
<usecase>
[User]-(Login)
[User]-(Logout)
(Login)<(Reminder)
(Login)>(Captcha)
</usecase>