Creating a variable or a state?

You will have to use the variable-function.

Yes, your idea is correct.

Make a variable (var_Part1) for the end slide of part 1 and adjust it to "1".

Then you'll make a new variable (var_Part2) at  the end slide of part 3 and adjust the variable also to "1"

The main idea is easy:

When has the user finished part 1?
When he arrives at the end slide of part1.

In technical terms:

If he has not visited the end slide of the part 1 the variable "var_Part1" = "0".

if he has visited the end slide of the part 1 the variable "var_Part1" = "1".

Then you'll go to the first slide. Change the initial state of every button to "deactivated"

Now you will have to make a trigger at start of the timeline.

Change state of button 2 at the start of the timeline when variable "var_Part1" = "1"

Here is a look at the conditions:

Then do this for every button.

Best regards
Ralf

Hey Carrie, you're on the right track. I wrote an entire step by step but then deleted it. I thought it might be better, and faster, if I just uploaded a file for you to dissect. There are many ways to do this but this one is fairly simple. The critical thing is paying attention to the order of your triggers. Look closely at the way they are ordered on the menu page. 

If you have any questions give me a shout. 

I was probably deleting my lengthy response when Ralf posted his (another way to do it).Maybe between the two of us you can come up with a working solution. Good luck. 

You could do it with states easily enough. Have all but the first button be disabled when the slide starts. Then create a trigger that changes the button to normal when the previous button is clicked.