You'll first get a chance to high five Claptrap in Sanctuary where he's located in the north west corner of the map. He'll also offer you the mission "Claptrap's Secret Stash".
You'll first need to complete the mission he hands over to you, which isn't very hard. Once done, turn it in and then follow him. He'll only walk around the corner to another NPC. Once he does, get close and you can then press to give him a high five.
Simply press and Claptrap will then put his hand up in the air. Now aim up and press to melee his hand for it to count as a high five. It may take a couple of goes to connect, but once you do, he'll put his hand back down and the achievement will unlock.
If you miss it this time, there's plenty of opportunities later on where you can ask for as many high fives as you like.
For those of you curious about the details to unlocking this achievement, OXM magazine did a feature on Borderlands 2 in this months issue (October 2012 ISSUE) and the key to unlocking this is printed very tiny at the bottom of one of the pages.
Not as easy as it seems fellas... apparently there's only two chances to get it without starting a new playthrough. I missed the first chance because I thought I had to press a button, but you have to melee his hand.
Not true at all. Once you've gotten to a certain point in the game, you can just walk up to Claptrap and hit Y which will activate the "Gimme Five!" option. He'll put his hand up and all you have to do is melee his hand. Super easy achievement. Womp womp womp.
To clarify a little bit without giving away any spoilers, I did this when Claptrap was wandering around Sanctuary after he had shown me his super secret stash. I got it in the middle of one of his dubstep sessions.
You people cannot miss this, don't worry if you couldn't get this at first, when you go forward in the campaign, in a mission you must upgrade clappy's software, when you finish this mission you can always ask for a highfive, I did it and now I have always the option to highfive, just melee his hand and achievement unlocked.
Can anyone else confirm what #29 said? I finished the "secret hiding spot" side quest and when the option for high-five came us I pressed it, and the little guy just ran away! Since he only says "sorry, I have no missions for you right now" it'd be nice to know I get another chance!
Actually, you can't miss this. As #29 said, after the Wildlife Preservation mission, you return to Claptrap. After you upgrade his software, he does a little routine and then says, "Give me five!" At this point point, aim your rectile on his hand and melee. Bingo. Achievement pops.
Ok I can also confirm that later on in the game you can high five him anytime you want. The Y button option will always be on him after you reach a certain point in the game. So don't worry people who missed the opportunity the 2 times he offers you to high five.