how do I get the Mehrunes razor quest to start?

[HomeInvasionTurbo]

in the original it popped up in the sewers? Whats going on

[Mansen]

The Remaster hid all of the DLC popups, because they would all popup and spam the player, since they're all included and enabled.

Simply go where it would normally send you, to start the quest. This goes for all DLC content.