If you're certain that you have the legal Steam version of the game then the only other thing it could be is a conflicting mod. What you need to do is remove all mods and restore the game back to vanilla, then only install the mission replay and see if it works. You can do this in two simple steaps:
1. Delete all files from the root of your SRTT install (where the game exe is) with the following extension:
*.xtbl
*.lua
*.str2_pc
*.asm_pc
2. Verify Integrity on Steam so it replaces any archives back to vanilla. You can do this by right clicking on the game in your Steam library, selecting properties, selecting the LOCAL FILES tab and clicking the VERIFY INTEGRITY OF GAME CACHE button.
Once you're back to vanilla, reinstall the mod by extracting all the included files in the archive to your SRTT install root like the instructions say, and try it again.