I believe to use iCloud you need to run at least Lion (10.7). (Only the Find My iPhone or Find My Mac parts of iCloud work with OS earlier than 10.7/Lion).
I think you can run Lion but you should check each of the following:
"To use Lion, make sure your computer has the following:
- An Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- Mac OS X v10.6.6 or later to install via the Mac App Store (v10.6.8 recommended); you can install without Mac OS X v10.6 by using an OS X Lion USB Thumb Drive, available on the Apple Online Store
- 7 GB of available disk space
- 2 GB of RAM
If your Mac does not meet these requirements, you will need to upgrade your Mac before installing Lion"
You have a Core 2 Duo. You can upgrade to 10.6 and then install Lion, or use the thumb drive approach mentioned above (thumb drive Lion costs about $70 I think). You need to check your RAM to see if you have enough.
Folks have reported that 4 GB RAM is really a minimum for a decent experience with Lion.
For Mountain Lion, see this ( http://support.apple.com/kb/HT5444 ) for what you need to have to install OS 10.8. I don't think you can use Mountain Lion (10.8) with your early 2008 Macbook.