Graygeek: you have to install all the folders supplied with the Odyssey Theme Pack. From the installation.txt file provided with the Zip:
1. Unzip the file, ensuring that the "Use folder names" checkbox is selected.
2. You should have four folders: gemini, arthurium, fu-default and ma-default.
3. Upload these folders and all their contents to the wp-content/themes directory in your 1.5 install.
4. Log in, go to "Presentation", and activate the theme of your choice.
Ryan Boren’s tutorial: https://boren.nu/archives/2004/11/10/anatomy-of-a-wordpress-theme/
The Odyssey themes all use the same templates so all the folders in the zip must be uploaded to your /themes directory, regardless of whether you plan to use all or just one of them.
I am in the process of developing some upgraded themes and plan to install the Theme Switcher plugin at my WP15 testbed, https://testbeds.webjones.org/wp15/
So stay tuned.
And P.S. I wish someone would go ahead and correct the misnomer, “Arthurium.” That theme was created by me from an original photograph I took of an ANthurium bloom. ??