To unlock developer mode left click on the bottom right corner of the screen in the main-menu, once you click it you should see a red text cursor start blinking if so type "devmode" and hit enter, it will then begin loading developer mode!!!
When in devmode the game is the exact same except there are a handful of commands to type in. to open the command text box hit "/" it will clear everything currently in the text box and add a "/" you can then type a command and hit ENTER.
COMMANDS:
/spawnmenu on - this will enable the spawnmenu where you can spawn every item in the game by clicking on the "SPAWN" button.
/spawnmenu off - will close the menu.
/notrees - will remove all the trees on the map
/alltrees - will enabled all the trees on the map
/spawn deadbody - this one is my favorite, everytime you click enter with this command typed in it will spawn a dead body ragdoll
/nowater - will remove all water.
/allwater - will enable all water.
/fast time - will speed up everything in the game.
/slow time - will slow down everything.
/normal time - will set the time back to normal.
/time day - will set the time to be day.
/time night - will set the time to be night.
/killall - will kill all the zombies currently in the game. (they will respawn).
/weather clear - this will set the current weather to clear (if already clear nothing will happen)
/weather rain - this will set the current weather to rain(if already rain nothing will happen)