The operation manual is available for free download from Samsung at this site:
The answer to how to lock and unlock is on page 15 of the manual. It's instructions are:
1. Press the '0' button, the display will show:
"PRESS 1- DEMO 2-CHILD LOCK"
2. Press the "2" button, the display will show:
3. Press the start button and the display will show "LOCK" then returns to the time of day.
At this point the microwave can't be used. To unlock, just repeat steps 1-3.
Hope this helps