Domain Mapping Plugin

I have a multisite with 3 domains living under a subdirectory. I am having a tough time getting them to map, even with the plugin. 2 of the domains need to come from a 3rd party, 1 of them is owned by me.

I want the mapped domains to carry the site. I.E. is in the address bar when mapped to local site (masking).

Can you please point me in the right direction?