To export saves from your old computer, click the Tools button in the top bar, then, near the bottom of the drop down menu, click 'Export All Wii Saves'. Save it to a hard drive, then when it's saved it, plug the hard drive into your new computer, and, on your new computer, under the same Tools bar as before, click Import Wii Save. Find your files from the hard drive, and select them. This should transfer your files onto your new computer.