[quote]Question I’ve go is in my old host , it would be root/http/Maindomain/Addondomains.
is it the same here or is it more
root/MainDomain
root/Addondomain
root/second addon domain[/quote]
It is more like you indicated above, with the actual path on DreamHost being:
/home/user/ (which is where you connect via FTP/SFTP)
/home user/MainDomain
/home/user/Addondomain
/home/user/second_addon_domain
Note that one DreamHost you can have multiple users at no additional cost, so you could choose to establish additional users, at which point your structure would be like this:
/home/youruser/ (which is where you connect via FTP/SFTP)
/home/youruser/MainDomain
/home/youruser/Addondomain
/home/youruser/second_addon_domain
and
/home/anotheruser/ (which is where another user connects via FTP/SFTP)
/home/anotheruser/MainDomain
/home/anotheruser/AnotherDomain
/home/anotheruser/andAnotherDomain
and so on, and so forth, “scooby-dooby-doo”! 
–rlparker