DOMAIN MAPPING plugin

I want to map my domain to main domain. I did multisite with subdirectory.
So I install domain mapping plugin into main domain. And then I add 2nd site to main domain, so my second site looks like this: http://www.maindomain.com/site2. Did DNS point to the appropriate IP address according to Domain Mapping suggestion. Can see the 2nd site from Dashboard, but not from Domain Mapping plugin, in this case from Mapped Domains. Did I do something wrong? Or it needs to be subdomain instead subdirectory? Thank you for help.