Spring Loaded flap
Your model is similar to the UK 638 which shares the same basic design as the previous UK 612.
The tools converter slides into the tools port on the side of the cleaner.
With the cleaner in the upright position, press down fully upon the rear chassis of the machine, near the handle. The rear suspension wheels will move closer to the body of the cleaner. Keep the cleaner in this position by maintaining your force. At the same time, fully slide the tools converter into the port (this allows the converter to penetrate the spring-loaded flap, close to the fan). Release your force upon the cleaner body (the spring-loaded wheel suspension will lock the converter in place).
To release the tools converter, apply force to the rear of the cleaner, withdraw converter. The spring-loaded flap will reseal the suction port automatically.