if you want steam to download the two files go to steam library right clock on avp in the panel where you selecting games and sellect there properties(it musnt be called exactly properties but something similar i do not have steam in english so i do not know how exactly it is called) it will open a small window select there local files. there you will have 4 options and the third one should be something like check something
. select the option and is will start checking your files if some files are missing it will download them.
to the problem with instalation of mods, there can be a problem in the mod but i cant say it on 100% without the mod.