Emilianomk
If you are really using Resolve 12.5 (and NOT 12.5.1) are you getting any messages when you launch Resolve about QTDecoder not being found or initialized?
You need to make sure you have the path that files are in, added to the Resolve Preferences - Media Storage area. Do you?
Once that's done, you should be able to select that path in the upper left Media Storage area of the Media page. Resolve should show you any files that it is capable of using. Do you see any files? If so, what happens if you select one? It should appear in the viewer in the center of the GUI.
If you are using Resolve 12.5.1 with Windows 7, and you are having issues with H.264 files, you should install this Windows update: https://support.microsoft.com/en-sg/kb/2670838