You need to specify the name of the condition type you want to use. ActiveContentExtension compares an extension, it does not try to do pattern matching, so you cannot use a * there. Also, the default action for a menu item where the condition fails is to make the menu item invisible. If you instead want to disable it, use action="Disable"
Example:
<Condition name="ActiveContentExtension" activeextension=".aspx" action = "Disable">
The available conditions are documented in SharpDevelop/doc/technotes/ConditionList.html (in SharpDevelop's source code download).