[Zope] Unable to add Plone Site

I've installed Plone 0.9.9 (with Zope 2.5.1, CMF 1.3, Python 2.1.3, Win2000), but am unable to add a "Plone Site" through the ZMI. The only Plone options that appear in the "Add Object" drop-down are "Plone Content" and "Plone Tool." The "Plone Site" option is conspicuously missing. Where am I going wrong? TIA, Aaron Gillette abg@comco-inc.com