The correct URL to get the access to the subsite1 is the normal way.
In other worlds if you have created a slave site in a specific
www.mydomain.com/subsite1, the administration is
www.mydomain.com/subsite1/administrator.
The information that is missing in your question are:
- Are you working on Unix platform and does the Symbolic Link is allowed ?
- Have you deployed the slave site into {root}/subsite1 and have you created this directory manually before deploying it when symbolic link is allowed ?