Installing the demo should have worked fine, assuming you ran the configuration wizard and clicked the "try it now" option (or whatever it is called) when you were asked for a license.
Is this a machine where you installed a SolarWinds demo previously? If so, that an be your problem - SolarWinds places multiple "hooks" into the software to block re-installs. So if you have a scratch server that you use for demoing software, and you've installed SolarWinds on this machine before, you should scrub the OS and then try installing SolarWinds again.