Hi,
You need to make sure that Windows sees the devices first-of-all. As long as the autoloader is detected, you don't need the drivers from HP's site installed.
If Backup Exec is already installed, you can run the Device Wizard to install the latest drivers. You might need to restart the services if you have attached the autoloader while the server is still on. Make sure you use the option to install Symantec drivers for all devices.
Otherwise, if Backup Exec is still to be installed, it will then detect the autoloader, and install the drivers.
Best practice would be to shut down your server if you still have to connect the autoloader. This will ensure that the devices are detected correctly.
Cheers!