Is your start game button on a layout already? If so, any instance variables you add to it, will be available for the event sheet to use.
Menus are all about IF and ELSE chains, relying on global or object variables to check for what you need.
Construct 2 is capable of making very complex menus, like in RPG skill trees (or even multiple skill trees). It is all down to user trial and error as well as experience to apply it.
For your one, you obviously need a global variable (GV) "StageCompleted" or something like it, to keep track of progress.
Then just add a check when you touch button.
ie. For the 2nd stage.
On Touch Object XXXXX
AND
StageCompleted >= 1
Goto Layout: Stage 2 (Doing this will ensure they will not be able to touch/trigger that button until their completed stage 1 already!!)
You can also have them greyed out unless they are up to a stage. Using the On Start of Layout to check for StageCompleted >= 1, then Change Animation of button for stage 2.
I use opacity for my ability and stage icons, I set them all at 50 default so they are semi-greyed out, and 100 opacity when players are up to that stage or upgrade.
Have a look at my Shop menu, took me a whole WEEK to get it work without bugs, lots of trial and error, but now if i need to do complex menus, take me a few minutes.
Example