Do I have to do something to start receiving mail in Poland? (moved from the US)

Odd, what makes you think they weren't stolen by the USPS or others?

because mail comes from lots of countries and it ends up 'different' when it gets here.

when those other countries send mail to countries other than Poland it's ok.

that tells me that the problem is in Poland and i have no problem saying that some Polish postal workers are thieving twats.

I've never, in my entire life, had anything tampered with or stolen from me by USPS. Ever. I've never heard of it happening to anyone else I know either.

Somehow, some way, when I go to Poland, it happens all the time. Oh, and other people say the same damn thing. As a matter of fact, when my mother in law realized that we simply didn't have the earings in the package she sent us, her immediate response was, "wiesz co!.....na pewno ktos ukradl to na poczcie," and went on to say how common it is for poles to steal stuff at their post offices. My father in law nodded his head in agreement.

A quick google search for "USPS stealing mail" reveals plenty of results, including news stories about USPS employees stealing mail.

The U.S. Postal Service received complaints from residents who suspected Dillard had taken their mail, according to court documents. USPS investigators planted mail that should have been delivered on his route.
On Oct. 8, 2010, they stopped Dillard after he completed his shift and found an opened birthday card that had contained a $20 bill that had been placed by the investigators.

Royal Mail do the same thing but they even use agent provocateurs, as a friend of mine nearly found out to his cost.

Go to Poczta Polska and complain about your post being stolen and see what they will do about it. Not so long ago there was a TV program which showed a video taken from inside a PP truck showing workers sorting through parcels to see what to steal: none of those workers were even officially reprimanded by PP.