Cape du Couedic is the same place you will see 'The Brothers' islands and Admiral's Arch
The seal colony can be found on the same point of land that's home to the arch and aforementioned lighthouse. From the car park for these attractions there's a detour to a dedicated seal-watching lookout where you'll be able to see them lying on the rocks below and playing in the waves.
However, I found that you actually get much better views from Admiral's Arch. The path over there is is lower so the seals are just beneath you, resting after spending the previous day searching for food.
There's at least ten seals in this picture, taken just outside Admiral's Arch, but most are hard to spot
An active seal
A tired pup
Even underneath the arch itself there's quite a lot, swimming in the whitewash.
Admiral's Arch is one of the biggest attractions on Kangaroo Island
As well as the other attractions in Flinders Chase National Park that I have mentioned, there are other animals to keep an eye out for too. I saw a koala by the Visitors Centre and an echidna by the Remarkable Rocks (and I was only in the park for a day). There's also waterholes where you might see a platypus, and kangaroos are here just like everywhere else.
Other places to see animals outside Flinders Chase National Park include include the koalas at Hanson Bay Wildlife Sanctuary and penguins at Kingscote or Penneshaw (where you can do a nocturnal tour) . There's also bees at Clifford's Honey Farm (the only pure strain of this type of bee left in the world) or marron at the Marron Cafe and local creeks. I managed to see seals just by visiting other beaches too; once at Pennington Bay, and twice at Browns Beach.