How are you trying to install them in FBI? The program doesn't download .CIAs so it shouldn't throw that error. It installs the archives from an SD directory.
Are you using the .3dsx app or the homemenu version? If it's the latter you've already installed some .CIAs since the guide has you install like 4 of them including FBI.
Walk me through your process. You should be able to just put the .CIA files on your SD card, open FBI, go to SD, find the .CIA file on your SD card, press "A" and choose "install"...... There is no downloading involved.
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Unless you mean you're trying to install programs from TitleDB inside FBI. In that case, check your network connection. FBI shows if you're connected to the internet or not, if you're not the list will fail to populate or download any apps.