ok, you have to use a MAS compresor, download it from rfactorcentral, utilities, or something like that, go and search, or you can use 3dsimed aswell!!!, then see a packed circuit to see how they are packed, gmt files in one MAs file, textures in another, and the rest of files outside, i mean Aiw, scn, etc..., note that in scn file you will have to edit wit wordpad and change some directories. All of this is easy try to do it, looking at other compresssed circuit and apply the same rules, for yours.
Regards