string to int: parseint(x)
where x = your string
increment/decrement int by 1: (x + 1) or (x - 1) where x = your starting int.
Combined: (parseint(x) + 1).
In your example: (parseInt($('.ui-btn-text').text()) + 1)
Edit* Additionally, in your jsfiddle, the line alert('NextPage function=' + parseInt(pager);
is missing an ending ")" it should be alert('NextPage function=' + parseInt(pager));