I'll tell you how it works with a mod I downloaded form another page (I hope it works for you).
You must add a line like this for every new plane in the Air.ini file (in the unmoded game it's in the "game root folder"\Files\com\maddox\il2\objects\, with the Ultra pack 2.0 its' in "game root folder"\-MODS\STD\com\maddox\il2\objects, Im not sure about your installation):
1) 2) 3) 4) 5) 6)
F4U-1 air.F4U1 1 NOINFO usa01 DESERT
1) is the name you will see in the list of planes
2) is the name of the Java class with the plane info*
3) is (I think) the side (1=alies,2=axis)
I'm not sure about 4),5) and 6) and I think you can copy them from any other line of that file
2)* the name of the Java class is something like this:"com.maddox.il2.objects.air.Airplane_name " (com.maddox.il2.objects.air.F4U1 for me), you must only copy "air.Airplane_name ". I'm not sure about the file where you can find it, but I'll show you where it was in the mod I downloaded:
As I said before, I hope it works.