I think the easy way is to it let make the index by it self. To do that use Array > action > Push.
If you do that, you have 2 options.
Option 1: Start with an empty array. (dimensions 1,3,1). To let it fill the array with an unlimeted amount of mouse coordinates.
Every tick
Action > Array > Push ... X axis .. value dont matter ... front
Action > Array > Set at XY > X=0 .. Y=1 .. value = Mouse.X
Action > Array > Set at XY > X=0 .. Y=2 .. value = Mouse.Y
Now the array fills, with the latest coordinates at index X=0
Option 2: Suppose you want to store a limited amount of mouse coordinates. Lets say 2000. Then you start with an array with dimensions 2000,3,1.
Now.
Every tick
Action > Array > Push ... X axis .. value really dont matter... front
Action > Array > Set at XY > X=0 .. Y=1 .. value = Mouse.X
Action > Array > Set at XY > X=0 .. Y=2 .. value = Mouse.Y
Action > Array > Pop > X axis ... back.