For maps, you simply drag and drop the map file into: ~\Renegade X\UDKGame\CookedPC\Maps\RenX\
When you're on the menu screen at the beginning, hit f5, then type `open mapname`. So you wanted to open Field, you'd type
open CNC-Field
For mutators, you'd have to put the downloaded mutator files into ~\Renegade X\UDKGame\CookedPC\.
Replace MyPackage.MyMutator with whatever the downloaded mutator tells you to put.