First, you need to create an Array and give it a name.
Then you can use the "Set Array" command to set a cell in the array.
If you use only the x position it will be a regular array and if you use the x,y position it will be a 2D array.
You can access the array using the [] syntax.
For example, if you want the 5th cell in a regular array with the name of arr1 you can write arr1[5]
And if you want the 3rd and 5th cell in a 2D array with the name of arr2 you can write arr2[3][5]
Here is an example of a regular array:
www.gamemaker3d.com/player?pid=01615828
And here is an example of a 2D array:
www.gamemaker3d.com/player?pid=01615844
Regards,
Cyberix3D team