put the demo in your tremulous/base/demos folder and open console in trem main menu than type. /demo nameofdemo
If you have tjw client it saves demos to
C:\Documents and Settings\Administrator\Local Settings\Application Data\Tremulous/base/demos
Not sure which you put them in to play them, I think with tjw client u use the documents and settings one. Also you can change this i believe its a variable.
Note-You have to enable hidden folders to find the 2nd. To do so in windows
open my computer go to tools, and click folder options, Its under the view tab.