After restarting Ardour, hit the small Log-button in the upper right corner of the main window. It should say something like (this is Linux, MacOS or Windows will be different):
</p>
-<pre>[INFO]: Loading 3 MIDI patches from /home/username/.config/ardour5/patchfiles</pre>
+<pre>[INFO]: Loading 3 MIDI patches from /home/username/.config/ardour6/patchfiles</pre>
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The added device should now show up in the dropdown mentioned in the previous paragraph.