Full credit to: JusticeEMU
If you missed your initial chance to high five Claptrap (like myself), don't worry.
Later on in the game, upon the completion of the quest "Wildlife Preservation" (story line quest, unmissable) Claptrap will request a high five, after his upgrade. Simply melee his hand, and the achievement is yours.
There's another chance even later, after completing the mission "Statuesque" in Opportunity. Return the mission to Claptrap, press (Y) on him to request a high five, and then melee.
EDIT: Adding a video from xblChoas (Goes along well with my text guide)