You need to configure it first before you can use it in TVH. Log in as the user you run TVH as, and the run @tv_grab_sd_json --configure@. Then you ought to be able to run it from within TVH.
(If you are installing from a package/repository, you may want to check that your distro is including @tv_grab_sd_json@. If you are building it yourself, check the READMEs to verify you have the prereqs, otherwise it won't build the grabber.)