To get the chair, you've got to 'like' the official Dunkin' Donuts Facebook page here and then head over to the section of the page titled "DD & The Sims Social".
Owning the lawn chair adds 250 points to your house value, which is a modest, but no small amount. It also sells for 15 Simoleons, which is chump change, but hey, free furniture! And lastly, the chair comes with a bit of extra animation.
Just click on the chair, and your Sim will get the option to "Sit" or "Drink Dunkin' Donuts Coffee". So kick back, and watch your Sim lie down on the chair to sip from a steamy cup.