You can't access iCloud through MobileMe. iCloud requires OS X Lion (10.7.2) or higher in order to set up iCloud on your Mac, which I don't believe can be installed on a G5. The system 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
You could, however, add your iCloud email account using these settings: http://support.apple.com/kb/HT4864.