without knowing what that particular addon is and you saying you downloaded it recently then it will undoubtably need resistance or probably version 1.96, which is the current version. Not alot of addons are made for 1.46 as most people have the resistance update. You need to have resistance before later patches work, some sort of computer trickery. As for soloutions you could either buy resistance (apparently easily avavible on Ebay, but some people would argue that) or you could open the mission files and delete the offending addons by hand, but then most missions need resistance and higher to run if they are using certain scripts etc. Hope some of that hepled. Try asking at th BIS forum as they know more about that.
GI-YO