nexus Media Importer doesn't show any files at all.
When you insert the OTG cable with your card reader attached, there should be something near the top of NMI that says: USB Device, and underneath that something like /dev/bus/usb/002/002 15.6GB FAT32 (the last part of that depending on the size and formatting of your card).
If you don't see that after pressing the scan button again, then the card reader isn't being recognized. Either the card reader isn't compatible or the OTG cable is bad. Also, make sure you insert the card into the reader before attaching the reader to your Nexus 7.
Oh, I almost forgot. Your original post mentioned formatting. Yes, the card or USB drive must be formatted as FAT32, FAT16, or NTFS.
And if you haven't done so already, try rebooting your N7.
If it does show the USB device, then, uh, it's something else?!
For photos, NMI can only read jpg and Canon or Nikon raw files, so if you have something other than those, I'd assume that they wouldn't show up. Also, you might want to try the developer's other app, Nexus Photo Viewer, to see if that works. It's a free app.
That's about all I can think of for now. Let us know if any of these ideas worked or if you found another solution.
Sent from my Nexus 7 using Android Central Forums