WRC9 OK. New to forum. Why won't 7-Zip extract AgentHex WRC9 mod .rar file?

[GRessex1]

OK. New to forum. Got all excited about AgentHex WRC9 mod so signed up and downloaded the .rar file. A quick Google established that 7-Zip should be able to extract .rar file but when I try, 7-Zip reports "not an archive". What do you guys use to extract downloaded .rar files?
Also, dummy questions but shouldn't there be some sort of instructions for when I have eventually extracted the files? Oh, wait, perhaps the instructions are in the archive!