Yes, please modify. Much easier on the eyes.
Now, what does it do, exactly? Does it crash to the desktop, give you an error message, etc?
You might be missing an addon. Look in the readme included with the mission to be sure you have it all. JAM does not matter unless the mission maker made the mission using JAM or an addon that requires JAM. Plus, what mission did you download and from where? If the steps above didn't help, you may have to tell someone from where you downlaoded it.
To put addons ito the game, you need to extract the PBOs to your addons folder. I suggest looking at Avon's FAQ.