In order to purchase upgrades for HP Photo Creations, you need to first create an account using the button on the top-right corner of the application. In case you've already done that, you should be logged in so that the tool can connect the purchase to your credentials.
To find out if you already have an account, use the Forgot my password button from the log-in menu and enter your email address.
If the account creation process or purchase itself don't work, your best bet is to contact the developers directly using this link: https://ssl.www8.hp.com/h41268/live/index.aspx?qid=27269
Lastly, the product's website also states that HP Photo Creations has some region restrictions so that may also be a problem.