If your page is blank there is a problem with your graphics driver because the window needs graphic acceleration. But there are multiple ways to redeem credits. Try the following:
Go into the in-app shop and select the category (instrument/Soundpools) and click on the coupon counter at the bottom. Then select the instruments and Soundpools you are interested in.
Click on the yellow credits button at the bottom. This will limit the Soundpools to those that you are eligible to purchase using credits. Select the more expensive ones. You are spending credits, not money. A credit is a credit - not linked to a particular price. There are no refunds.