ADMT
At the time of writing this Blog;

ADMT 3.1 - 32 Bit version. ( Will not install on Windows 2008R2 )
This installer has SQL 2005 Express Edition ( with necessary service pack integrated into the installer )

ADMT 3.2 - 64 Bit Version. ( The first & only version which will install on Windows 2008R2 )

SQL 2008
There are many download options available on the Microsoft website,
* Express Edition & Enterprise Edition
* 32 Bit & 64 Bit
* With & Without Service pack integrated.

Your mileage will vary depending on what version of SQL, Windows & ADMT you use.

For the purpose this migration, I used ADMT 3.2 on a windows 2008 R2, member server with SQL 2008 sp3 Express Edition ( x64 )
This was installed in the target forest.
Password Export Server was installed on a Domain Controller in the source forest.

How to Check the version of SQL ?

Type these commands in a cmd prompt:

sqlcmd -S.
select @@version
go

Additional Reading: Link, Link