I haven't used a Fadal but I find the Haas control pretty easy to work with. It would be best if they would send you to either the mill or lathe training class. If you're near a local Hass Factory Outlet, they may provide that training, free of charge. Or they may have online training setup through the web. There are tons of tricks to using this control. Having a little background first will actually make the training much more worthwhile to you than if you were green to it.
You're aware that you can highlight any field in the offsets page, then punch in a number, right? Of course you are. Now the answer to your question: you know that any number in the display can be added to or subtracted from, by typing another number on top of it and pressing Write/Enter:
So if your Z offset reads -12.457 after using your edge finder, you could type -0.100 at the control, press Write/Enter and it will subtract that from your offset, changing it to -12.557.
I use 123 blocks in my vise to set offsets on the mill. This method will work for all three axes in that case.
To change an offset value to be the actual number you typed in (not added or subtracted) I think you type the number, then press F2. I'm pulling this from my head so you should double check that. I don't do it very often but I use this method for changing tool diameters or the tool radius in the lathe.