You also have to make sure that when you create the movieClip, you select the "export from Actionscript" tab - it's under the 'advanced' tab when you're creating a movieClip. You can also right click the movieClip you wish to add from your library and click on 'roperties' - that will give you the same menu. Also No_Thumbs' code above is for Actionscript 3.0, if you're still coding with actionscript 2.0 it will be a little different.
Right click on your item on your library and select Linkage, mark "Export for ActionScript" and give it an Identifier, it works pretty much like a instance name but for library
Then you can use attachMovie, like this
YourItemName = attachMovie("YourItemIdentifier", "TheNameofthisNewInstanceofYourItem", the depth of your item, { _x:YourX , _y:YourY });
You can't have two movie clips on the same depth or they won't work, you can solve that by using _root.getNextHighestDepth()
For the new name you can either use a variable or use the same number as the depth on the name, thus making each of them different, "TheNameofthisNewInstanceofYourItem"+_root.getNextHighestDepth()
Do note that both AS2 and AS3 were quoted in this thread... using both, no matter which version of Flash you're using, will generate quite a few errors...