Go into your "My Documents" folder. Find the "Planescape.lua" file. Add this line to the file

-SetPrivateProfileString('Program Options','Debug Mode','1')

Save the file. When you are playing the game you should now be able to hit [Ctrl]+Space and see a black box appear. Type in the appropriate code in the box and then hit enter.

Note: As of 2.0 the old code prefix "CLUAConsole:" no longer works. You can instead just use "C:"

Example: C:"code"()

Code - Effect

C:AddGold(x) - Adds x gold to your party total.
C:CreateCreature(x) - Creates the monster x. x must be in quotation marks.
C:CreateItem(x,y) - Creates y number of item x. If y is ommited, one x will be created. x must be in quotation marks.
C:ExploreArea() - Reveals map of the current area.
C:MoveToArea(x) - Teleports selected characters to the area x. x must be in quotation marks.
C:SetCurrentXP(x) - Sets the experience points of the currently selected characters to x. You can't go higher than the experience cap of 2950000, however.