Updated for 7.0
On the left-hand side of the screen in the navigation bar, select Substitute(s). This will take you here:
Select the name of the substitute member listed under Open Request(s).
Click the box with the icon of two people located to the right of the name. A pop-up window will appear that looks like this:
Click the Approve with Existing Account button to approve the substitute member.
These are sample names.