you are setting the element data of "thePlayer" then you try to get an element data value of the root element. if you change the serverside line to
setElementData( getRootElement(), "someValue", 100 )
it should work but if you want to have different players with different element data values you need to get the player element you want to set the element data for.
it depends much on what you try to do but let me give you an example
server
for i,aPlayerElement in ipairs(getElementsByType("player")) do
setElementData(aPlayerElement,"bulletsfired",0)
end
client
function addtobulletcount()
setElementData(getLocalPlayer(),"bulletsfired",tonumber(getElementData(getLocalPlayer(),"bulletsfired"))+1)
end
addEventHandler("onClientPlayerWeaponFire",getLocalPlayer(),addtobulletcount)