I fixed my issue with the admin panel not being displayed. There were 2 things that caused this issue.
1. My ftp program was not loading some files from the admin directory. I ended up loading these files using my hosting company's ftp program instead of Dreamweaver.
2. I replaced the configure.php file that I was using with my old version 1.3.8, with the dist-configure.php file that came with 1.5.1.

I am now moving on with my upgrade. Hopefully others might find this useful, since I banged my head on this for a few hours! Save you the trouble.