Hi i am not the most technical man in the world so some basic help would be appreciated, i have an imac, macbook, and apple tv and i use all of them regulary, so i dont have to keep updating my imac and macbook by using a external hard drive, and so i can access both, can anyone teach me this ?

Also my apple tv is connected to my mac book, and i have tried with my imac but when i go to add devices in my itunes on my imac its not there ?

Any suggestions ? i am using the same wireless router for all 3 ? my reception is fine ? but could do with help on all of the above,