The Pix-ASA migration tool has been withdrawn.
You should be able to copy the configuration from the PIx and load it into the startup-configuration file of the ASA. When you then reload the ASA the cli parser will perform the migration for you.
Most people advise looking at your NAT setup carefully and taking the opportunity to rebuild it using object NAT as the parser sometimes doesn't make it as nice as you can by thinking it through and doing it via manual configuration.
A lot depends on how much / what you ave configured on the Pix. If you have site-site VPNs with pre-shared keys make sure you have those available (more system:running-config). Also any remote access VPN profiles or X.509 certificates would need to be moved over manually or recreated