How can I ensure that DaVinci Resolve will detect and/or load my .mp4 / .avi files? New user of DaVinci Resolve, explain-like-I'm-five. =)

Have installed and followed instructions for setting up Resolve 12 on my Windows 7 16gb ram computers with GeForce 760 gpu.

Initially, Resolve would not recognise my .mp4 / .avi files, only finding single image files. The .mp4 and .avi files will load and run in VLC and / or Blender.

I restarted the program, at which point it could find the files - but the thumbnails for the files were red with an exclamation mark and would not load. There were a couple which displayed correct thumbnails, but on clicking on the files the thumbnails turned to red with exclamation mark - and would not load.

Restarting my computer and reloading Resolve returned me to my initial condition: Resolve would not recognise my .mp4 / .avi files, only finding single image files.

These are files of mostly less than 100mb each, some only lasting for a couple of minutes.

Does anybody know why Resolve can't detect or load my files?

--Rev
From your post I am not sure why Resolve wont recognize your files...but you should transcode your footage anyways before ingesting into Resolve. 

Even if you start with highly compressed source media, you should transcode to something less compressed, and that will put you in 4:2:2 at least vs working with 4:0:0. ProRes or DNxHD codec will give you files that are much less compressed, and will yield the best possible results in color correction.