i found this via a7oogle in a thread (http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&topic=27906&gonew=1) :

//Make a Pretty Sky
skycube = (ENTITY *)ent_createlayer("Sky_2+6.tga", _VAR(SKY | CUBE | VISIBLE), _VAR(0));

in c# it would then be

ENTITY MySky = EngFun.ent_createlayer("Sky_2+6.tga", (Var)(1 << 17 | 1 << 20 | 1 << 14), (Var)0);

In the next version this will be fixed and i will add an enum for CUBE SHOW and the other flags and i will also add a property for SKY which i somehow forgot !

In fact, you may also write it, like you will be able to in the next update of the wrapper (But you have change the enum FLAGS2 to public enum FLAGS2 in ENTITY.cs as workaround until the next update arrives ^^):

ENTITY MySky = EngFun.ent_createlayer("Skycube+6.tga", (Var)(int)(ENTITY.FLAGS2.SKY | ENTITY.FLAGS2.SHOW | ENTITY.FLAGS2.CUBE), (Var)0);