FALLOUT 4 - p90 mod location (read description)
near beginning of game, incase you weren't aware
Mod by: shinyhaxorous
Update: There seems to be a lot of people not seeing the gun, but just to let everyone know that shinyhaxorous himself said it would be in the spot on the video when replying to someone with the exact same question. My advise is to redownload the mod and if that does not work than start a new game. Thank you for understanding, and watching.
Thank you