Simcoe hops are placed in a stainless steel hop canister and placed in the keg at kegging.
The mango extract is added into the keg at kegging.
This method of dry hopping in the keg happens right at the bottom of the keg by the dip tube. Without stirring, the beer right at the dip tube can end up tasting overly hoppy. You may need to stir in the extracted hop oils in the keg by inverting the keg several times on day 2 and/or day 4 after kegging. This will help disperse the hop extract throughout the beer in the keg. If the beer tastes overly hoppy, you should definitely do this.
You can add more simcoe hops than 2 ounces if you want more dry hop flavor.