On the first line you are simply telling what the name of the variable is that you are using for the unique-identifier. In this example it is "uid".
<package version="2.0" unique-identifier="uid" xmlns="http://www.idpf.org/2007/opf">
Then you are defining what that variable is:
<dc:identifier id="uid">4183454533</dc:identifier>
You can easily use the ISBN like this:
<package version="2.0" unique-identifier="BookID" xmlns="http://www.idpf.org/2007/opf">
<dc:identifier id="BookID">ISBN [isbn # here]</dc:identifier>