Just in case peeps missed it before, here's a clip & instructions of how to install DLC / bonus maps, if this content is unlocked for you.
Once extracted, the game directory will look something like this:
WindowsNoEditor β‘ NSUDEVR
βͺ Binaries
βͺ Config
βͺ Content
βͺ Plugins
Extract the DLC maps and put the extracted folders into the Plugins folder, so your Directory should now look something like this:
WindowsNoEditor β‘ NSUDEVR β‘ Plugins
βͺ BeachIsland
βͺ SubwayPack
If everything's in the right place, the maps should be selectable from the Levels tab.