How to add an alternate airfield
Once you have made a plan, from the Map Page, tap the 'Plan' Button. The plan plan pop-up will appear. Now, tap on your destination. A page will appear which has a vertical wind profile, segment details and a bunch of other stuff. Down that list is an item called 'Alt. Direct To'. Tap on that and enter the ICAO of the divert airfield.
When you navigate to the the plan sheet now, on the fuel sheet you will see the ICAO entry in the divert column. The ICAO will have a slash and a number next to it, which is the leg TIME. The column next to it is of course the fuel amount.
You can also type the alternate directly in to the Fuel Page of the plan sheet direct if you wish; simply tap on the Alternate field, type in an ICAO and press enter. OZRWY will then work out the divert time and fuel.
NOTE: The divert fuel is calculated using the wind and altitude from the last leg. If this number isn't conservative enough for what you desire, you can overtype the fuel amount, but note this will change the Min column from ICAO/tt to tt. i.e. you will have to remember your diver or put it in the notes column.
Another way to do a 'full up' alternate option, is after you have planned and submitted a FN, add the divert location as the last leg of your plan. That will enable you to put the desired detailed figures in, such as wind and altitude, and plan for the air work required of an instrument approach. If you are very tight for gas, this is the best way to do it.