No plex

I tether jailbroke my apple tv2 on 6.1.4 installed aTV black and under maintenance there is no plex client please help

Plex is not part of ATV Black - it is just that the Maintenance app offers a convenient way to install it.

I do not use Plex myself but I am guessing that the fact it is not shown probably means Plex is not known to be compatible with the version of firmware you are using. You might want to ask in the Plex forums to be sure.