For the collaboration to take effect, make sure that you update the game.
Here are some of the details via Capcom Unity:
An otherworldly creature has arrived in the New World and we need a hunter with special abilities to help the Research Commission hunters solve the anomalies it has ushered in. Enter Geralt of Rivia, The Witcher.
As part of the Free Title Update (PS4 version 6.00 / Xbox One version 18.104.22.168), you’ll get to play as Geralt from The Witcher 3: Wild Hunt in a unique experience that brings out the best of both game series. We’ve even called upon the original voice actors for Geralt to play his part again in Monster Hunter: World!
The first part of this collaboration puts you in Geralt’s boots and gives you all-new mysteries to solve and gear to craft with the Special Assignment “Contract: Trouble in the Ancient Forest”. After you complete the assignment, the Geralt α full armor set will be available for crafting, as well as the Sword & Shield weapon Witcher’s Silver Sword for your Hunter. And for your Palico, the Nekker α full armor set and Cursed Staff α will show up in the Smithy’s list.
Once you’ve cleared this Special Assignment and crafted your shiny new gear, start prepping for the second part of this collaboration: an even tougher challenge with even more rewards! The limited time (Feb 15th – Mar 1st) Event Quest “Contract: Woodland Spirit” will be set for HR50+ hunters, but this time it’ll be open to multiplayer, so get your squad ready and take on this ancient threat. Rewards include the Ciri α full armor set and a matching set of Dual Blades. Good luck, hunters!
Monster Hunter: World and The Witcher 3 are available now on PlayStation 4, Xbox One, and PC.
Check out the launch trailer below: