I'm tearing my hair out trying to figure this out. I've read that you can copy an APK file to the SD card and use the browser to activate the installer to install it with this:
file:///sdcard/yourApp.apk
(from: https://stackoverflow.com/questions/4268803/android-how-to-install-apk-from-sd-card)
But, I cannot seem to get that to work with external_sd card. I've tried
file:///external_sd/yourApp.apk
I keep getting the following message:
"Data connectivity problem - The Web page contains an error."