To start off.
When the player chose the first level.save the level num(just do a add int 1 to variable as first level mostly will be num 1(unless its not in your case.give it the number it should be instead) in a variable(global).
Than what you can do is,with the next level button,add 1 to the current int storing that in the variable and than read that variable with the load level num.
Player shoces level1[this is num1 for example]>add INT 1 to variable levelnumber.
Player finished level and klicks the next button>Get variable levelnumber and add 1INT upon the existing int(1+1=2) save that in the variable.>Get levelnumber variable in the"load level num action"under "Level index"
hope this is a bit more clear xp