To staking postoken it must be on the postoken website wallet or can be in myetherwallet??
It can be any ethereum wallet that accepts erc-20 tokens standard. But anyway remember that you need to use mint() option - stake manually.
Is there any tutorial how to use mint?? I have 500 postoken in the mew wallet but nothing staking

It's quite straightforward... login to MEW, go in the send tokens menu, set a reasonable min GWEI price on top right (cheap but also safe for the tx to go through), send 0 POS to yourself (your current eth address holding the POS), set gas limit 200'000, send the transaction.
I followed your instructions but i left gas limit as it was (20,000). But i think it didn't go through (because it shows a red exclamation mark next to the Tx). Right after i sent that transaction, i went to the coinage calculator and it showed a reward of 45 POS. But right after that, i tried to do exactly what you said to do (this time with 200'000 gas limit), and it went through. But when i go to the calculator, it now shows a reward of 0 POS. Did i lose that reward, or can i still get it? If so, then how? Thanks