Server is up, now with password to keep people who haven't read the OP out. Password is at the bottom of the OP.
How though? Like, what would you put in?
modVariable Brick [Variable Name] RandomNum [5]
ifVariable Brick [Variable Name] Equals [1]
This way it will pick a random number out of 5 and if it is 1 then it does something (onVariableTrue) and if it is not it does something else (onVariable False). Since 1/5 is 20% that means there's a 20% chance it will hit 1 and a 20% chance that onVariableTrue happens.