I almost accidentally saw this question and decided to provide an answer in case you're still didn't find one in the documentation.
To open files with associated programs you need to add the following command to your
filetype * open %c
i keys will work for you as expected. See help on
:filextype in the documentation.
This doesn't work out of the box, since different systems have different
open commands and the default behavior is to open all files with Vim (for example, xdg-open for Ubuntu 16.04, as opposed to the aforementioned "open", which is specific to Mac). Maybe new version will contain a set of
open commands for different systems in sample
vifmrc file, so it will work from the beginning.