What do you mean you "can't select the folder of the game"? Where are you trying to select?
By default, Ace opens the last project you were working on. So you should just need to run Ace and it ought to be there.
If it's not, you should be able to click the Open Project button or choose it from the menu, then navigate to wherever your project is saved and select it from there.
Finally, you should be able to go to the project's folder in Windows Explorer and double-click on the Game.rvproj2 file (its description is RPGVXAce Project).
If none of those work, tell us where you saved your game when you first created it, and if you've done anything since then like moving it to a new location, whether it was on an external drive or USB or you put it in dropbox or something, and whether you've reinstalled Ace since you created the project.