What version? The ACS migration tool is no longer supported with ISE 3.1 and later due to all versions of ACS being EOL now. You should be able to download the older version of the tool from CCO under the ISE downloads.
do you have a URL, I've checked previous versions and it's AWOL.
To be honest all I want to do is export a list of users & passwords to import into ISE, it doesn't need to be via the migration tool. I just can't figure out a way to get all the usernames and most importantly the passwords from the ancient ACS box.
We have a running ISE cluster doing all the auth for TACACS & corporate WiFi access. The ACS is legacy running a guest WiFi and for ease of migration in a healthcare setting I just want to move the users on-mass rather than experience the pain of re-registering every single one.