You can access the Availability page on mobile by tapping the Menu icon in the top-left, then Availability.
If you haven't added any availabillity patterns yet, you'll be presented with the 'Add Availability Pattern' form straight away when you open the Availability section.
Filling in this form is slightly different on mobile - you can enter a Title and select Effective Dates as normal. However, instead of entering your availability for the whole week at once, you can select individual days from a drop-down and set your availability from there.
Once you've done that, just tap Add.
If you already have patterns entered, you'll be presented with a list of your availability patterns. Tap on a pattern to edit it, or tap the blue Plus button to add a new pattern.