WoW:Using OnUpdate correctly

first of all you have to have a main frame where you define the onUpdate function that will be called on every update. it is important, that this frame has the tags stated below ... hidden="false" toplevel="true", otherwise your function will not be called

this is in your XML file.

 <Frame name="MyAddon_MainFrame" parent="UIParent" hidden="false" toplevel="true">
   <Frames>
   </Frames>
   <Scripts>
     <OnUpdate>
       MyAddon_OnUpdate(arg1);
     </OnUpdate>      
   </Scripts>			
 </Frame>

this is in your LUA file

 MyAddon_LastUpdate = 0;
 MyAddon_Update_Interval = 1; --the update interval in seconds

 function MyAddon_OnUpdate(elapsed)
   MyAddon_LastUpdate = MyAddon_LastUpdate + elapsed; 	
   if (MyAddon_LastUpdate > MyAddon_Update_Interval) then    
     MyAddon_LastUpdate = 0;
     --do Stuff here
   end
 end

for '--do stuff here' you should insert your own sourcecode that should be run every 'MyAddon_Update_Interval' seconds. of course you should replace all function / variable names ( MyAddon_ ) with a unique string representing your addon.