Your alias popupmenu has to be a custom identifier that returns something. In addition, you don't need to loop for the Nth token using %x - as you already have a N: $N

Code:
alias popupmenu {
  if ($1 isnum) {
    var %rooms = #IRC-SHACK,#TEST,#MIRC
    if ($gettok(%rooms,$1,44)) { return $v1 : join $v1 }
  }
}