"Oops!
ERROR 709
The account is unknown. An account needs to be part of at least one transaction (sender or recipient) to be known to the network."
go away...
Thank you for trying out the NEM client. We would all appreciate if you would refrain from such language and attitude.
We are here to help.
Do you have any XEM? Are you a stakeholder? Have you bought some XEM?
If not - upon opening an account (after creating a wallet) in your client you will have an address with 0 balance.
You may start harvesting when you have at least 10000 XEM vested.
PoS/PoI (proof of stake/proof of importance) cryptocurrencies work significantly different than PoW (proof of Work) ones.
Please take a few minutes and visit our official web site http://nem.io/ where you can find more information about the cutting edge technology built into NEM and how to use it.
If you have any questions after you've read tutorials and faq (you may access them from the top of our official web site), feel free to ask anything you want...
Good luck!