What's the exact name of the upgrade package file? If you downloaded the latest from the support.arubanetworks.com download page, it should look something like: AMP-7.5.5-x86_64-cvs.tar.gz
If this is the case, then the upgrade command should be:
# start_amp_upgrade -v 7.5.5
If you're still having problems, please send the return of:
# ls /root