Do you mean the big My Applications icon beside HyppTV & My Media ?
I have no idea why doesn't work for you, try a reboot but clear default in its settings first before reboot, after all working fine then you can set it as default

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Is the usb debugging enabled before the update ? If yes, you can try to install from pc connected to your box with adb command.

adb install xxxx.apk

Not so sure as I don't test this but used to do it for phone .. long time ago. Will try it later when I'm home.

If possible, later tonight I can upload Settings.apk so you can unlock unknown sources for future installation.

Update : unable to test .. I don't have male to male usb cable.