The Apple Help page you came here from says:
'Can I buy more storage if I need it?
Yes. If you need to upgrade your storage to a larger amount, you can purchase one of our standard storage upgrade plans at the regular price. Please note that when you purchase a new storage upgrade, it will replace your complimentary storage plan. If you cancel the upgrade at a later date, you will return to the standard free 5GB storage plan and will no longer have access to your complimentary storage upgrade.'
In that event this paragraph should apply:
'What if I already upgraded my storage plan prior to this extension?
If you upgraded your storage plan before September 30, 2012, we have automatically extended your upgrade at no additional charge until September 30, 2013.'
Which is a little ambiguous but could mean either than you should have the free upgrade and the purchased 10GB, or that the purchased 10GB won't kick in until next October. What does your Account details say exactly? And if you check your credit card have you been charged?
If the situation does not meet your expectations you will need to contact Support: if you currently happen to have AppleCare, either because you recently bought Apple hardware or have paid to extend the inititial period, you can contact them here:
You will need the serial number of the covered hardware.
If you are not covered by AppleCare, then - in common with other free email services - there is no free support (MobileMe Support is no longer available) and you may be asked to pay a fee, but in the circumstances you should be able to get them to deal with this without charge.