Q: How to get my new macbook pro to access my itunes library from an external drive without using migration assistant

Start iTunes and immediately hold down the option (alt) key.  Point it to the library on the external. 

It's probably the same for iPhoto.

Sound like you have just the media folder, not the entire library in a self-contained form. You may be able to extract the library folder <User>/Music/iTunes from the Time Machine backup and then persuade it to connect to the external drive. If possible the external drive needs the same volume/device name it had on the previous system.

tt2

That won't get your your "library".  Your library, in essence, is (was?) a file called library.itl in your iTunes folder.  This is a database with a listing of which media files you have and where they are, long with playlists, ratings, and a bunch of other things (which artwork goes with which album).  If all you have on the external drive are the media files then when your old computer died it took the library.itl file with it.

What are the iTunes library files? - http://support.apple.com/kb/HT1660

More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management

What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html

iTunes 9 [and later]: Understanding iTunes Media Organization - http://support.apple.com/kb/ht3847

Image of iTunes folder structure Dec. 2011 - https://discussions.apple.com/message/16923545
see also:
Image of folder structure and explanation of different iTunes versions (turingtest2 post) - https://discussions.apple.com/message/13025536 and https://discussions.apple.com/message/17457605

If you want your full library listing back then you will have to recover it from your bakup drive (which is why you have it).  Alternatively you can create a brand new library file, add the media to it, and start new.

This worked for me.  I had tried changing the iTunes preferences to point to the time machine directory where my iTunes content was, but it was not finding the library.  Holding the Alt key on iTunes startup allowed me to select the same directory where my content was, and iTunes found the library.  Thanks.