I did some more research and this is how you should do it. This instruction is for Bitcoin Core but it should be the same for your wallet, give it a go.
Let us know if this works for you.
Run Bitcoin-Core
Select Help (to the right of Settings)
Select Debug Window
Select Console
If you encrypted your wallet, unlock it by entering walletpassphrase "YourLongPassphrase" 600 next to the > input box
Next to the > input box type importprivkey <bitcoinprivkey> (Note that the private key must not have spaces, remove them if they are included in your backup)
Example: importprivkey 5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS
Source:
https://bitcoin.stackexchange.com/questions/5941/how-do-i-import-a-private-key-into-bitcoin-core