Migrate builds on SQLAlchemy, so you should install that first.
You can get the latest version of Migrate from the cheese shop, or via easy_install:
easy_install migrate
You should now be able to use the migrate command from the command line:
migrate
This should list all available commands. migrate help COMMAND will display more information about each command.
If you'd like to be notified when new versions of migrate are released, subscribe to migrate-announce.
Migrate's Subversion repository is at http://erosson.com/migrate/svn/
To get the latest trunk:
svn co http://erosson.com/migrate/svn/migrate/trunk
Patches should be submitted as Trac tickets.