Try visiting
mysite.com/subsitename/admin/
This should give you the administration page for that subsite. Click on Applications and you can install applications under that subsite.
Choose parameters to set the default template for that subsite.
The OpenACS request processor checks a database table called site_nodes to map a URL to a package directory and file.
You can learn more about the request processor if you are interested: https://openacs.org/doc/openacs-5-2/rp-design.html
