zsnes is written partly in 32-bit assembler, so it isn't available from
the amd64 versions of the Debian package repos. You need to add the
32-bit ones as an available option in order to get access to 32-bit-only
packages.
Try
$ sudo dpkg --add-architecture i386
$ sudo apt-get update
and then try the install again. I suspect it'll succeed this time.
--
The Wanderer