A power adapter. It's a very popular router. See here (Amazon). Third row down, number 3 on the right.
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Delectronics&field-keywords=power+adapter+linksys+wireless+g+router&x=0&y=0&ih=37_2_1_0_1_0_0_0_0_1.70_409&fsc=-1Did the cable guy say exactly HOW you would be better off with a Netgear router? Thought not :) Look, Netgear makes good stuff, I have some but your router gets good reviews, is fast and easy to set up.
I'm assuming you have cable internet as many ADSL modems double as a router. If it turns out you need to purchase a new router, here are a couple of links to reviews. The OzCable guy one is particularly helpful as you can fill out a checkbox list of your requirements and it will come up with suitable routers.
http://www.smallnetbuilder.com/wireless/wireless-reviewshttp://www.ozcableguy.com/router_reviews.asp