The Arduino Uno uses a specific vendor ID of 0x2341 and product ID of 0x0001

If you modify your Arduino Uno to use a DIFFERENT vendor ID or product ID, you
can add your vendor/product combination to the files/ids.txt text file
according to the example shown.  Additionally, this driver MAY work with
generic ACM USB Serial Devices.  You would need to manually add the
vendor ID and product ID for these devices as well.  For ACM devices that
are modems, consider using or modifyig the 'umodem' driver.

You will need to load the uarduno kernel module before connecting the Arduino UNO
via USB to your computer.  The command to manually load the driver is:

  kldload uarduno.ko

To automatically load the driver on startup, add the following line to
your /boot/loader.conf file:

  uarduno_load="YES"