If you bought the Legendary Edition then Season Pass 1 and 2 should be automatically added to your Steam account and It's content should be immediately available in-game after completing Tutorial race.
To check if they are added to your account open Steam Library, Right Click on the game, choose "Properties" then look at "DLC", SP1 and SP2 and all the DLCs included with them should be visible there.
To check in-game you need to open the game, go into Downloadable Content and look at Premium Cars if most DLCs have green mark.