Hi Cheryl,
If you've mixed files from the two versions, the easiest way to fix it is to install J2.5 in a new directory and add all your modules and plugins in. Then change the database connection in configuration.php to point to your original data base and see if it's all working. If it works, move the original install and replace it with the new one.
You could spend many hours comparing directories and trying to track down 1.5 files in a 2.5 install. So a clean install connected to the original database usually works.
Cheers,
Ed