import receiving address

I send some Reddcoins to my wallet before it was synched. I deleted everything except my wallet.dat file, and downloaded Reddcoin Core again, and now its all synched.
But my receiving address isn’t in this wallet. I found a beautiful tutorial from gary0318 to import from the debug window , but it doesn’t work for my address or my private key. I get these errors:

Private key for address R… is not known ( code -4 )
Invalid private key encoding ( code -5 )

What can be the problem? Thanks!