In your wallet, there is a link that says "permissions". Under that you will need to click the button next to "posting key" that says "show private key". The key that shows there is the key you should use to log in for everything except managing your wallet.
I did. It doesn't recognize it.
Are you sure you used your posting key? Not your password or owner key, they won't work.
How can I get a posting key ?
In your wallet, there is a link that says "permissions". Under that you will need to click the button next to "posting key" that says "show private key". The key that shows there is the key you should use to log in for everything except managing your wallet.