With the latest firmware, the user interface is new...
You can find WDS on Advanced---System Tools---System Parameters page.
For detailed instruction, you may refer to user guide(page 95) for C2600.
http://www.tp-link.com/res/down/doc/Archer_C2600(UN_V1_UG.pdf

Thanks Mavis, found it and now I'm connected :)

I have two other issues now, perhaps you can help...

1. Why am I only getting a max TX/RX of 450Mbps when both the D9 and C9 are set to 600Mbps on Wireless N. On the Wireless AC side, I'm getting the full 1300Mbps. (checked using InSSIDer). I have connected the WDS on the 5Ghz band.
2. When I connect to the C9 as an AP, I can see my full network but when I log on to my D9 (primary modem/router) I can't see the C9 or any devices connected to the C9 on the network.

1. Where did you see speed 450Mbps? Provide a screenshot? What's the wireless speed of wireless card on your clients?
2. Did you check DHCP Clients list on D9? Only devices connect to it directly by DHCP will show up there. You may try to connect any device directly to its LAN port or wirelessly.