I've right clicked on the game in steam, but I didn't see an install option, nor did I see it in the DLC section. The game told me I had to purchase it when I went to addition content, but Steam told me I didn't. And when I went to the redeem content place, all I saw was the wins, actions, friend, achievements and broadcast.
Ubisoft has since fixed the issue redeeming the Season Pass and you should now be able to redeem the Season Pass CD Key using Uplay's "Activate Product" menu. You can find the Season Pass CD Key in the CD Key menu under Trials Fusion in your Steam Library, assuming you bought it or a bundle containing it on Steam.