I'm recording a demo for benchmarking with Nexuiz, but when I play it back (using the command ply <name of demo>), it doesn't show me the FPS scores. So how do I play back a demo and have it show the FPS scores?

playdemo and ply are just normal commands to play/watch a demo
The command timedemo DEMONAME is what you want.
playdemo and timedemo normaly use the Nexuiz/data directory.. ply is an alias that does the same as playdemo but uses the Nexuiz/data/demos directory. I use this alias

alias tdem "timedemo demos/$1"

to have something easy for timedemos..

just enter timedemo demos/<demonamehere>