They will show up in machineTypes as (e.g.) x86_64-linux:local instead of x86_64-linux. This is to prevent the Hydra provisioner from creating machines for steps that are supposed to be executed locally.
To start hacking on Hydra, run:
$ ./dev-shell
$ ./configure $configureFlags --prefix=/opt/hydra
$ make
$ make install
Or, if you just want to build from source (on x86_64-linux):
$ nix-build -A build.x86_64-linux release.nix
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