Wireless modem router question?

For the past 2 years i have been using a netgeat CG814WG Modem Router and my younger brother bought a Net gear N600 Wireless Dual Band roughter thinking he could extend the signal of the CG81v...he tried to connect it up but hasnt succeded. Basically what i'm asking is it possible to use the N600 to extend the signal. If so can you please explain how.