Unfortunately you cannot pay to retain your storage on Snow Leopard. If you know someone with a compliant Mac, or and iOS5/6 device, or can go to an Apple Store and get a 'Genius' to help you, you would be able to extend the storage there. The instructions are here:
http://support.apple.com/kb/HT5527
Scroll down to 'How do I purchase my current plan?' and expand the section applicable to your computer or device.
Otherwise you would have to upgrade at least to Lion, see below, but you have only two days to do any of this. Do you really need more than 5GB? - you can't have been using it much with Snow Leopard as only email can be used. If you do nothing your storage will revert to 5GB and as long as you have less than that you won't notice any difference.
The minimum requirement for iCloud is Lion 10.7.5 (Mountain Lion preferred): the iCloud Preference Pane does not appear on earlier systems - the MobileMe pane appears on Lion and earlier but is non non-functional - you cannot now open or access a MobileMe account.
To make use of iCloud you will have to upgrade your Mac to Lion or Mountain Lion, provided it meets the requirements.
The requirements for Lion are:
- Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- 2GB of memory
- OS X v10.6.6 or later (v10.6.8 recommended)
- 7GB of available space
To purchase it you will have to ring Apple at the number given at the bottom left of this page.
The requirements for Mountain Lion are:
- OS X v10.6.8 or later
- 2GB of memory
- 8GB of available space
and the supported models are:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- Xserve (Early 2009)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
It is available from the Mac App Store (in Applications).
You should be aware that PPC programs (such as AppleWorks) will not run on Lion or above; and some other applications may not be compatible - there is a useful compatibility checklist at http://roaringapps.com/apps:table