Download it and put it into the main game folder, it is a well known fault
from the help site
It seems that in some cases the file is not being transferred over from the cd correctly and the file shows up as 0kb. The rts.dll file should be 882kb.
To resolve this problem, please download the attached 'rts.dll' file and save it in the main Forgotten Battles folder (C:\Program Files\Ubi Soft\IL2-Sturmovik Forgotten Battles) as well as the ubi.com folder (C:\Program Files\ubi.com).