How to get new migrations to show up in the Migrate UI
First, the function *_migrate_api() in *.migrate.inc should include the $api with the key api of 2, and the key migrations of the array for your migration.

If you have newly added your migration, drastically changing the code etc, you perhaps need to make Drupal recognise the hook you have just created in your migration module; a guaranteed way for that is to disable once and (re)enable your migration module.

Then you need register your migration.  If you use Drush,  

drush migrate-register --group=YourMigrationGroupName


would do a job.  After Flusing all caches (drush cc all etc), you will see the migration in your migration UI (hopefully!).

I am not an expert, but the above is based on my recent experiences.