When you run ServiceStack with MVC you should typically define ServiceStack to pick up the requests to a custom location such as api
using <location path="api">
.
See here for setup instructions:
Therefore ServiceStack won't handle requests to other locations and as such the location path <location path="admin">
will be outside the path handled by ServiceStack, so the Authentication of your elmah.axd
will be handled by IIS. So IIS will authenticate as per usual.