This may be a little more obvious to experienced steemians, but I recently ran into this problem. I also noticed in my quest to solve the issue, that I wasn't the only one. Here are the steps to logging in to DSound, which is the only platform I've had the issue with, that requires a steem account.
If you attempt to log in using your regular (public?) posting key, this will be what you see:
The solution is pretty simple. First go to your profile and click "wallet"
Click "permissions"
Click "show private key"
Copy (and preferably save) your private key.
Finally, paste your key and you should be able to log in.
