I use this plugin in an multisite environment and it works quite well. I can edit domain mappings for all site except for the first site, i.e. with id 1.
1) If I try to add a domain map using the settings under "Network Admin" it does not create an entry for it
2) The domain mapping menu is missing for my first page. All other website have the entry.
In my case I added the entries manually to the database using SQL and it works like expected.