I think it didn't work because I forgot the "s" try this:
-- Not tested
addEventHandler ( "onPlayerSpawn", root, function ( )
if ( getTeamName ( getPlayerTeam ( source ) ) ~= "Zombies" ) then
setElementModel ( source, 280 )
outputChatBox ( "You are now a zombie.", source, 255, 0, 0 )
else return end
end )