Should I be able to generate multiple addresses? Option is grayed out in my wallet - 0.99 GUI.

No with the current version of XDN Client you can generate only one address.
The main reason is - you actually need only one address, because of XDN privacy protection.
E.G. In case of Bitcoin you should use multiple addresses to save your privacy, DarkNote is designed to save you privacy by default.
But later we will introduce 1.multiple addresses generation 2. Multisignatures.