For the export HTML templates: Select the template in the export window as if you were going to do an export and press the 'Edit' button at the bottom of the window. That'll create a copy of the template, append 'My' to the name to designate that it's a template you edited and save the copy in your Bookpedia templates folder. Now you can open the template with any good text editor and start customizing it by changing or adding fields, etc. Once you're done, save the file and you'll see it appear in your Export drop down menu among the other templates. (If you use TextEdit to make changes, be sure to use the 'Open' command and tell the application to 'Ignore rich text commands'.)
For the info view HTML templates: To customize an info view template, ctrl-click the DVDpedia application icon and choose 'Show package contents/Contents/Resources/WebViewFiles/en' and select the template you'd like to customize.
To save your template, create a new folder titled 'InfoTemplates' inside your home folder under ~/Library/Application Support/Bookpedia and place the template in there. For any images, you have to create a folder inside 'InfoTemplates' named 'Images'. If you were to keep the template inside the application with the other info view templates, the template would get overwritten every time you update the program.
For the print HTML templates: Ctrl-click the DVDpedia application icon and choose 'Show package contents/Contents/Resources/Templates/' and select the template you'd like to customize. (All the printing templates are pre-fixed with 'Printing' so they're easy to distinguish.) Now you can open the template with any good text editor and start customizing it by changing or adding fields, etc. To save your template, create a new folder titled 'Templates' inside your DVDpedia data folder and place your template in there. Remember to keep the 'Printing' prefix so that it'll show up in the printing feature. For any images, you have to create a folder inside 'Templates' named 'Images'.
To find out more about customizing templates, take a look at the Help file under 'Working with Bookpedia/Customizing info view templates' or 'Exporting/Customizing HTML templates'.