Open the NIB that is in the Fluid package (make a backup first) with InterfaceBuilder.
In the main NIB control window (the one that shows the objects in the file) select a table view of the elements by clicking on the button in the right hand scroll bar (the button that looks like a listview, not the one that that looks like a set of squares). Now you should see the hierarchy of the Fluid config sheet. Expand the "Panel" item and then expand the "Content View" item. Now, double click on the "Invisible Controls" item. Without clicking on anything else, hold the shift key down and hit the UP ARROW on the keyboard 6 times to bring the controls into position.
Save the NIB and exit. Configure Fluid and enjoy editing the flowX and flowY of the liquid. 0.0 will give you the equalized settings that are the default. Try between -1.0 and 1.0. They will be noticed most with the "water" setting in the "Model" tab.
FYI, the other two controls edit how the grid for particles is setup. sizeMult should be between 0.0 and 1.0 and tells how much of the screen should be covered by the particles at startup. density is how many particle emitters are spaced up and across and the screen.
[robg adds: Fluid is a fairly amazing and robust screensaver, and worth the download time if you've never seen it before ... I have not tested these modifications.]

