Slide the card into the gap between upper edge of the key cap and casing. (For the small "arrow up" key it's on the right side, for the "arrow down" key it's on the left side, for the "return" key it's on the left side.)

Gently lever up until you hear two clicks (or up to four for longer keys like shift or space bar). They indicate that the cap is released on both notches.

Be careful not to use too much force. All parts of the MacBook keys are quite fragile with tiny plastic notches that can easliy be broken.