With advmame on a PC (and I assume you are on a PC..the rasp pi is NOT going to run area51 no matter how hard you try.) The chds have to be placed in it's own directory..at least by default.
/home/user/.advance/image/kinst/kinst.chd for example
place the chd in /home/user/.advance/image/area51/ so it's full path is
/home/user/.advance/image/area51/area51.chd to the chd
make folders if needed...
So what i mean is
Open your advance folder
Make a folder named image if one is not there
Make sure your folder is name the SAME name as your chd file (without the .chd extension of course.)
Place your chd file into the folder area51
(as mediamogul has said)
Now place that folder into the media folder
so now your chd is at /.advance/image/area51/area51.chd