You can right click in the “To” register to get the “Paste transaction” menu, or select Edit → Transaction → Paste Transaction(s). Note that if the menu item is greyed out, then you haven't yet done the copy transactions operation in the “From” register.
Have you got the transactions selected like described in
Once you do that you right click on any of those transactions:
And like I said, you can also select: Edit → Transaction → Copy Transaction(s)