I am having the same issue. I install invoice ninja on my website, download the mobile app to login and it request that I enter the API_SECRET. I went through the document http://docs.invoiceninja.com/en/latest/mobile_apps.html and i can find the " .env file".
Pleased I need the file name where the (.env file) so i can work on it. steps, name and location of the file to access (.env file) will be appreciated