Did you migrate from a full MobileMe account or from a Family Pack sub-account? the latter did not get the complimentary storage (because their iDisks were only 5GB anyway).
If you had a full MobileMe account, check on your Mac - System Preferences>iCloud - and see whether that shows the upgrade. If it doesn't, you probably 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: in this case you should ask them to deal with it without cost (and hope they agree).