Pomagac, pomóc

Pomogać w czymś...pomogać w pracz domowej...

Please, when is the instrumental (or locative???) used with pomogac/pomoc w, and when is the genitive used?

That is probably a stupid question, but I don't have any grammar book. :(

But if you need a grammar book ,check this site out :)

polish.slavic.pitt.edu

Genitive? Never.
Dative is when you say whom are you helping, for example:
I'm helping mother - pomagam (D: komu, czemu?) matce
Help me - pomóż (D: komu, czemu?) mi

Locative or instrumental is when you are precising the action:
I'm helping with the housework - pomagam (w kim, w czym?) w pracach domowych
Help me with this - pomóż mi (z kim, z czym?) z tym

"Pomoc" is also the noun in Polish "help", e.g. "Dziekuje za pomoc." = Thank you for your help.
"Pomagac" is the imperfective verb form.

Marek

two errors here:
pomagać w pracy domowej

Marek, the noun is "pomoc", the verb (in perfective form) is "pomóc"

That is what he said. Ta pomoc is help but czy ty mozesz mi pomoc is the verb in the perfective case.