There's a bit of a trick to attaching the cartridges to the handle:

  1. The cartridges should be left in the plastic case.
  2. With the rounded end of the case facing towards you, bring the razor handle towards your chosen cartridge (Fig. 3)
  3. Without touching the release button* push the handle until you hear a click (Fig. 4)
  4. Simply lift the handle until the cartridge pops out.
  5. Get shaving!

Attaching-Razor-Cartridges

 

* The button should only be used when detaching the cartridge (Fig. 1 & 2).