where is ~/.vim folder? and how to install themes?

~ always is your home folder (/Users/rororo for instance). If ~/.vim exists, that's where you'll find it. Unless you've used vim before, the folder likely will not already exist, so to create the folder, open Terminal.app and type

cd ~
mkdir .vim

Assuming you downloaded ColorSamplerPack.zip to your Downloads folder you can then proceed with

cd .vim
unzip ~/Downloads/ColorSamplerPack.zip

to unpack the content into the folder. A restart of gvim may be required afterwards.

On second thought, the same result can be accomplished by just opening Terminal.app and executing

unzip ~/Downloads/ColorSamplerPack.zip -d ~/.vim