I Cant Play Natural Selection

gumms5

<div class="IPBDescription">So annoyiing!!</div> I've installed Stream and then Installed NS. After I installed it I tried to play but it says it's an unkown program and it needs to be opened by another program. Which program do i use and if it's stream how do i put it under my play list..? (note: I think i posted something like this before but my computer froze up after posting it and im not sure if i posted it or not).