Most of the times the infinite loading screen means 2 things: uses 9.6 encryption, or as the others said above, it's out of region.
If the app/game you want to install is not released in your region, coverting the .3ds to .cia makes it region free, iirc.
Otherwise, if it uses 9.6 encryption, you'll need to cryptofix the .cia. You can search for "cia cryptofix", and it should explain how to do it.