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.