ill tell you what you need to do:
the zombie volumes, the ones that are in the play area have a min dist to player of 600. the ones that are outside of the play area need to have a min dist to player of 1.
600 in play area
1 outside of play area
next, the KFlevelrules is a actor: go first to Actors - Info - Replication Info - KFLevelRules.. place that in your map. when you look at its properties there is a WaveSpawnPeriod, the default value is 2. i suggest setting it to .5, or .1 (if you really want the zombies to start pumping out!)..
the skybox isnt major but id suggest looking at other maps to see their skyboxes, maybe copy paste one into your own map (make sure it has the same fog settings as inside of your map). the skymesh is what goes into a skybox to make it look like sky.
the terrain isnt a major deal either but it would look alot better.