I don't believe there is an option there to use a custom private key as he asked.
Yes there is, under "wallet details"
You can enter a private key in format: WIF, WIFC, HEX, B64, B6, MINI, BIP38
For example, the private key for Base6 address "1425342302314253423023142534230231425342302314333323023142534230231425342302314
25342302314253423023" is "5JMBJfgXCPpxB5HjiVJB4YvZ2wv4M6z3wqbTjtPqFKMCEC4E2jL"