-- the universe won't wait for you
Plasma was set to debut in a two-parter in Hiun City and showdown with Team Rocket inc. Sakaki but those episodes were never aired due to the catastrophe in Japan. So they put off using Plasma until now.
As for N, going by the latest aired episodes, he seems to have been raised to be Plasma's King/Reshiram's hero, but failed. Or something. The goddesses saved him and they've been running from Plasma but N has decided that saving innocent Pokémon from their deeds is better than just hiding away.
idk man you should just... watch the episodes. And next time use this thread for general questions rather than a specific episode discussion thread.