Seek'N'Loot is part of the main storyline. You get this quest in the lifeguard tower from Sinamoi after completing "A piece of Cake".
Sinamoi wants you to check the two gas stations and bring him two packs of juice. Quest Difficulty is marked as Hard, due to the amount of Zombies you will encounter here, including a good amount of Thug Zombies.
Take the car and follow the waypoints on the minimap to the first gas station. Clear the zombies, and locate the pack of Juice behind the counter.
Place the pack of juice in the designated slot on the back of your pickup, and head towards the next gas station. Drive through the tunnel. Shortly after leaving the tunnel, you will spot Laurie Weaver standing on the side of the road. She will give you a sidequest, Crash. Help her with the quest (which is a pretty easy sidequest) and continue towards the gas station. This will actually be a pretty long drive.
Once you get to the 2nd gas station area, you will see sparks of electricity on the ground, and a new sidequest called Waterdance will be unlocked. You need to complete this quest before advancing. (see the waterdance quest walkthrough)
Once you have turned off the electricity, locate the ladder in the area where the electricity bolts were. Climb up to the gas station roof, then find the glass and smash it and jump down into the gas station. Break open the door, take the juice and place it in your truck. Drive back to the lifeguard station.
Back in the lifeguard tower, take the packs of juice out of your truck and place them in the garage where both grey outlines are (common mistake: If you leave the juice packs in the back of your truck, and go talk to Sinamoi, he will ask you "Did you check both gas stations?")
After you have placed the juice packs in the designated area, talk to Sinamoi upstairs and this will give you a good xp reward and complete the quest.