They are refering to another adapter. The DIR-615, if running stock firmware, can not act as a adapter. The internet connection is coming from the WAN and going to the WLAN and LAN. It cant take a internet connection from the WLAN and bring it to the LAN. Plus you cant have drivers via a ethernet. Well you can, but their your NIC drivers, the drivers that run your Network Interface Card.
My question is why do you need it to do that? Would would you need to connect a wired desktop to something connecting to a wireless access point? Networking? Printer? Game console? The router acts as the hub(general way, not technically) between the PC and the access point, so they are connected that way, so trying go use a wireless adapter on the desktop to connect to the access point is redundant.