Reading old iTunes library on new Macbook Pro

I'm hoping someone can help me, because I can't find anything on the interwebs ... I just bought a Macbook Pro (2015) and am upgrading from a late-08 Macbook. A long time ago, I moved my iTunes library to an external hard drive because it had outgrown the space on my macbook.

Now I'm trying to connect my XHD to my new computer and access the library, but can't. I hold Option and open iTunes, then try to point it to my library, but get the error message: "An iTunes library could not be found in the folder "iTunes". Please choose a different folder." Which makes no sense, since THAT'S where my library is! What am I doing wrong? If I connect my XHD to my old machine, it reads it just fine.