I have had the same problem and got a response back today:

In order to obtain the additional 2000 sounds, please proceed as follows:

1. First of all, please make sure your program is up-to-date via the online update function in the "Help" menu.

2. Deactivate the "Easy mode" via the upper, left-hand option for this within Music Maker.

3. Click on the small Catooh button all the way on the right-hand side in the lower part of the screen (mediapool).

4. Log in with your Catooh account details or create a new Catooh account, if you don't have one yet.

5. Now, pick the  Music Maker MX +2000 Sounds Package", put it into your Catooh shopping basket and proceed to the checkout.

6. During the payment procedure, put the given coupon code (G2...) into the appropriate field and select to redeem it. This coupon code came with your boxed product or was sent to you via e-mail.

7. In the final step you will be able to download the package. If you pick the option in the Catooh window to apply the media after download, the soundpool styles will automatically be installed in Music Maker. Otherwise, you will need to add them manually in Music Maker via the "Add styles" command in the soundpools manager.

The G2 code is on the back of the cd rom sleave with the main serial number.

why they did not make this clear is beyond me.

Becareful with the code as you need to input it exact ie G2-12345-ABCDE-67890 for example. eg all capitals, numbers etc..... It took me 5 attempts to download. Note this is not the key to the sounds.