Right, if the listing ad said;
NETGEAR ASDL WIRELESS ROUTER / MODEM BRAND NEW
Then yes, the listing is WRONG. Because what you have there is a ETHERNET ROUTER WITH WIRELESS ACCESS POINT - not a modem.
Now the seller might have been confused here also so it's POSSIBLE it's not his fault.
What you have is a router - you plug it into an ADSL point (in the home situation this would be a broadband modem - in offices chances are it would just be a network port on the wall which comes from the main "modem")
The routher takes the internet feed from the modem, and splits it. Built in is a 4-port enthernet switch allowing you to hardwire 4 computers to you broadband, whereas before you could only have one PC hooked in. It also has a WAP (wireless access point) allowing you to connect computers, laptops, PDS's etc to it wireless'ly aswell.
So I take it you have broadband at home already? What modem do you have, and how is it connected to the computer? If it is connected via ethernet cable then you're fine - just set it up using the diagram in the pics you posted and you are sorted. If yours is a USB plug into the ocmputer then you need one of these;
A USB > Ethernet adpater. Plug the USB cable from your modem into one end (plug types vary's so get the correct one for your type) and from the other end you can plug in a cat5 ethernet "patch" cable to connect to your router (the point in the pic marked INTERNET)
Then connect the router to the PC/laptop via another cat5 patch cable, or wirelessly if your PC/laptop is wifi enabled.
HTH's!!!!