There are two ways to play single player one of them is easier to do. Megaton doesn't support user maps in single player for the expansions only multplayer

A. get Eduke32 and launch it with the Nuclear winter grp (feel free ask questions if you have problems)

B. Take the map out of the workshop folder rename it E2L1 and place into your gameroot/add ons/nw folder it will replace level 1 so make a copy of it :)
I suggest playing sp in Eduke32