Aligning vertical centers:
Aligning horizontal centers:
How to align the boxes so that the space between them would be equal?
You'll have to do a little maths. There's no way to say align objects vertically based on how they're horizontally aligned. If, for e.g., your boxes are 10mm and the gap 2mm, and the first box starts at 0x,0y then the second box starts at 12, the second at 24 (3rd at 36, etc.). Just calculate where the last box sits (whether you have just 3 or 300), position it there by changing it's x and y coordinates , select all the boxes in the row/column and then align to centres. Then do the same vertically.
Now while that's an okay answer, I was - of course - not thinking.
The better way to do this is to select all the objects in the Row or Column, click a second time on your first item in the row - to make it your Key Object (it'll then have a 'thick' selected blue border and everything will align to this one). Then in your
Align Palette type in what the gap between the objects should be. For e.g. 2 mm
This way you don't need to do any maths.
(That should help for your question if the boxes aren't equal sizes.)