You get the C.E.L.L. camo for certain weapons by reaching certain levels, to get it for the SCAR I think you need to reach level 50 (and then maybe fill your xp bar for that level).
I think the answer may lie in the 'challenges' for your Scar weapon :p Use the weapon more (get more kills) and you may open up more for the weapon in question.
I'm rank 28 on my first reboot, and so far i have the C.E.L.L. skins for the Scar, Feline and hammer, so no skins for the K-volt, or Scarib just yet, hopfully on my next reboot.