where are the rats? do you know what their main food source is? for example most of our rats are around the chicken shed, so i would bait with chicken feed as they are already eating it, if your not sure then i would recommend snickers bar or peanut butter as they go crazy for both.
as for camo, rats are brave little buggers and will explore most things if they smell food. i never feel the need to camo mine. and the only reason i would hide mine is if non targeted spices could get in, such as birds. (with the trap being outside)
hope this helps and good luck.
atb