Most Unity features are available when running in demo mode... about the only thing you can't do is some digital networking functionality such as AMIS and Cisco Bridge links to remote systems... everything else is open. Just the number of users and number of ports is limited and you can only record 15 second messages to ensure it's not being used in a production environment (or if it is, folks at that site are very brief and to the point).
To get a full license, you'll have to purchase one. There is no timed license for "try before you buy" functionality on Unity at this point, although in 4.0 coming out later this year such a plan is being considered.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.