You can find this Group Policy setting here:
In the Group Policy Management Editor console, expand Computer Configuration, expand Policies, expand Administrative Templates, expand System, click Net Logon, and then double-click Allow cryptography algorithms compatible with Windows NT 4.0. Based on (http://support.microsoft.com/kb/942564)
Another thing I found was that that Target Domain Account you are using to transfer the computers over need to have local admin privileges on the systems you are migrating from the source domain and the ADMT System needs to have access to the computer. (IE: Firewall opened for at least that machine).