I just installed Libreto+Emulation station on Linux LXLE (Based on Ubuntu 14.04). I installed latest SDLMame 14.04 Trusty and had same problems. When loading Mame roms it complained that files where missing.
Here’s what I did to fix it.
I installed latest Mame bios pack. (Google it)
I added the Mame rom path to the Emulation station config file
I added the mame rom path to the Mame config file
And that did it.
For some reason Mame wants the rom path in both config files.
Hope that helps
One more thing:
It’s possible that the Mame Bios pack you use may also cause mame to complain about missing files so try different bios packs as well.