Puzzlingly though this stanza seems to contradict the relativistic nature of pagan beliefs somewhat by dividing the world into absolutes (i.e. good and bad). A relativistic view means that a good act is only good when viewed from a certain perspective. If that same act is viewed from another perspective then it is perceived as bad.
So perhaps the stanza is talking about taking pleasure in acts that are good from 'your' perspective rather than taking pleasure in acts that are evil from 'your' perspective. If that's the case then it is contingent on you being aware of both good and evil so that you know what you are doing. Which is like a passage from the Bible that "we should be wise as serpents yet gentle as doves", inasmuch as people should know evil, which means studying evil, and thinking about evil, yet keeping distanced from it and affirming good rather than evil.
Ráðumk þér Loddfáfnir
en þú ráð nemir
njóta mundu ef þú nemr
þér munu góð ef þú getr
en lát þér at góðu getit
I advise you, Loddfafnir,
to take advice;
you would benefit, it you took it,
good will come to you, if you accept it:
 never be
 glad in evil,
but let yourself be pleased by good.