Fog - Go to setuplevel.fpi and write down as a first line after the headers :state=0:fog=1
For mist,you'd need v1.17+ to have particles and use the smokepuff decal.
Right click on it,set it's main script to
decalparticle.fpi and
change the values of Random Motion X and Z to something around 300-400,also change the
life of the decal to 5 and the transparency to 3.Play around with the values to achieve a better effect,this is off-top,so it won't be perfect.

PC Specs:Windows 7 Ultimate 64-bit,Intel Core i7 960 @ 3.20GHz,NVIDIA GeForce GTX 480,12GB RAM,2x Western Digital 997GB HDD