[Wasabista]

Both sentences are wrong, as you've figured out.

In principle "de" means "with/using" and "ni" is a general locative or dative particle.

Saji de shiru o tabeta. I drank my soup with a spoon.
Hon no naka ni wa mushi ga ita. A bug was inside the book.


But Japanese uses "de" when describing an action, in places where we would say "in," not "with":

Watashi wa Ikebukuro de hataraite imasu. I work in Ikebukuro.
O-sento de kaminoke o aratta. I washed my hair in the public bath.