Several bonus items require a special code to be redeemed on your save file, which are only available with certain editions of the game. You can find any codes available to you inside the "Extras" folder in the game's install directory.
Kael's Axe, the Fire Horn, and the Bardic Brian Booty bonus items are all available in the Adventurer's Guild shorty after the start of the game. For the Red Boots, search the Grand Plaz area in Skara Brae Underground - you will find them on a charred corpse in a hanging cage.
The Lute of Compulsory Cavorting is a special crowdfunded unlock that is in the game for everyone, no codes or unlocks required. The lute is available via vendors, victory drops, and in a crate across from the Adventurer's Guild in Skara Brae Underground (but requires solving a puzzle to get to).
Likewise, the Bardic Brian Skin is available to all players. To use it, create a custom character and select Bard as your archetype. You'll have Brian as one of the Bard portrait options.