Sorry to hear about this problem, but what you're experiencing there is not a general program fault. It strikes me as either a temporary server problem, or some kind of local/caching problem related to the in-app store system. I just checked, and everything seems to be working fine on my end when it comes to acquiring soundpool/collection content. Thus, my recommendation would be to simply give this another shot. If it's still not working, please open the Music Maker program settings (hotkey "P") and use the "Clean up Store" option in the "General options" tab. Once the program has restarted, make sure to log back into the in-app store system with your MAGIX account and attempt this purchase again.
If you're still not getting on, and you require further troubleshooting, or you want to cancel your order, you'll need to contact MAGIX customer service directly: