Many guides online mix up these two games. The Ctrl + Shift + F9 shortcut is highly effective for I.G.I. 2, but it is often reported to work for original I.G.I. 1 installations as well. PC Cheats - IGI 2: Covert Strike Guide - IGN
If entered correctly, all missions from "Trainyard" to "Finding Priboi" and the final "Maeva's Revenge" will be selectable in the mission list. The Configuration File Method (Permanent Unlock) how to unlock all missions in igi 1
Once you have unlocked the missions, you might find that the difficulty is still a major hurdle. To help you get through the later, more complex levels, you can use these internal developer cheats: Type allgod at the main menu or during gameplay. Many guides online mix up these two games