Keep in mind that it is
VERY RISKY to enter your private key on a website. If anyone gains access to your private keys, they can
AT ANY TIME NOW OR IN THE FUTURE spend any bitcoins that are received at the address associated with the private key.
Thus, you might want to do it yourself ->
https://en.bitcoin.it/wiki/Wallet_import_formatKeep in mind that bitcoin's base58 encoding uses a different order (1-9,A-Z,a-z) than the "regular" base58 (1-9,a-z,A-Z) you might encounter or be familiar with.