What do you mean with read only access from linux? The howto is for anabling Windows workstations to store their data on the samba server, its not an NFS howto for connecting linux workstations to the server.
If your files are marked as root:root owner, then you used the wrong user in windows. You will have to create a new linux + samba user, the connect with this username and password from windows. Not with the Administrator user. The Administrator is the root user on linux, it is only used in the process of adding your workstation to the domain to authorize you once as admin.