The .ace compression format is rather legacy, but you still find the occasional old file compressed in it, and p7zip-full won’t extract ’em. There’s a package in Ubuntu 10.04 called unace, and you’d expect this to do the job – only it doesn’t. It’s version 1.2 which is absolutely legacy…. I mean seriously, it’s archaic – and it won’t extract “modern” .ace files.
So, to get this all sorted, just head on over to Softpedia and grab unace v2.5 (which itself was built in 2003!), extract the tar.gz, and then either use the unace binary from where it is or sling it in your /usr/bin/ directory for convenience.