When assigning spaces to blocks on the block page you must drag and drop your space over the desired block. This drag and drop sequence can take a little getting used to so here are a couple of pointers and graphics to help walk you through it.
In order move a space you must click and drag the space name. Notice in the graphic below how as soon as a space is clicked it becomes highlighted in a bubble. If any space or block name has the mouse cursor's focus then it will become highlighted like this.
In order to drop the space into a desired block simply hover over the block name while holding a space. While hovering over this block both the block and the space now have focus and are highlighted. You can also see that the mouse cursor changes from an arrow to an arrow with a box under it to indicate that you are holding a space over a block you can drop it in.
When moving the mouse while holding a space you will notice that other spaces can take focus also, just like blocks. When doing this the mouse cursor changes, but this time to a circle with an line through it. This indicates that a space cannot be assigned to another space and if you drop the space here it will not move from it's original location. If both spaces are together you will need to drop it in the corresponding block instead.
This same circle with a line through it cursor appears when holding a space away from any other items, over empty area on the page. Since there is no item with focus below the cursor it will indicate that you cannot drop the space here and if you let go of the space it will not move from its current block.