[Suicide Junkie]

Leave the replacement box unchecked, since you aren't trying to overwrite the old turn.

Browse to your zip file, and then hit the "Upload replacement turn" button.
Hrm, that is a bad label for the button. It really should be "Upload New Turn" to be clearer.
The checkbox is for specifying whether this new turn replaces the old one (replay turn #X using this new file), or is just a plain new turn (turn #X+1).